Adds Observation Granularity

- introduces observation levels (same as in logging)
- extends the current API to allow passing of ObservationLevel

fixes gh-3442

Author: marcingrzejszczak

Diff for: docs/src/test/java/io/micrometer/docs/observation/ObservationConfiguringTests.java

@@ -76,10 +76,12 @@ void observation_config_customization() {
             .observationHandler(new DefaultMeterObservationHandler(meterRegistry));
 
         // Observation will be ignored because of the name
-        then(Observation.start("to.ignore", () -> new MyContext("don't ignore"), registry)).isSameAs(Observation.NOOP);
+        Observation ignoredBecauseOfName = Observation.start("to.ignore", () -> new MyContext("don't ignore"),
+                registry);
+        then(ignoredBecauseOfName.isNoop()).isTrue();
         // Observation will be ignored because of the entries in MyContext
-        then(Observation.start("not.to.ignore", () -> new MyContext("user to ignore"), registry))
-            .isSameAs(Observation.NOOP);
+        Observation notToIgnore = Observation.start("not.to.ignore", () -> new MyContext("user to ignore"), registry);
+        then(notToIgnore.isNoop()).isTrue();
 
         // Observation will not be ignored...
         MyContext myContext = new MyContext("user not to ignore");

Diff for: micrometer-observation/src/main/java/io/micrometer/observation/Level.java

@@ -0,0 +1,28 @@
+/*
+ * Copyright 2024 VMware, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.micrometer.observation;
+
+/**
+ * Observation Level.
+ *
+ * @author Marcin Grzejszczak
+ * @since 1.13.0
+ */
+public enum Level {
+
+    ALL, TRACE, DEBUG, INFO, WARN, ERROR, FATAL, OFF
+
+}

Diff for: micrometer-observation/src/main/java/io/micrometer/observation/NoopObservation.java

@@ -27,7 +27,7 @@
  * @author Marcin Grzejszczak
  * @since 1.10.0
  */
-final class NoopObservation implements Observation {
+class NoopObservation implements Observation {
 
     private static final Context CONTEXT = new Context();
 

Diff for: micrometer-observation/src/main/java/io/micrometer/observation/Observation.java

@@ -90,6 +90,60 @@ static <T extends Context> Observation start(String name, Supplier<T> contextSup
         return createNotStarted(name, contextSupplier, registry).start();
     }
 
+    /**
+     * Create and start an {@link Observation} with the given name. All Observations of
+     * the same type must share the same name.
+     * <p>
+     * When no registry is passed or the observation is
+     * {@link ObservationRegistry.ObservationConfig#observationPredicate(ObservationPredicate)
+     * not applicable}, a no-op observation will be returned.
+     * @param name name of the observation
+     * @param level observation level
+     * @param registry observation registry
+     * @return a started observation
+     * @since 1.13.0
+     */
+    static Observation start(String name, ObservationLevel level, @Nullable ObservationRegistry registry) {
+        return start(name, Context::new, level, registry);
+    }
+
+    /**
+     * Creates and starts an {@link Observation}. When the {@link ObservationRegistry} is
+     * null or the no-op registry, this fast returns a no-op {@link Observation} and skips
+     * the creation of the {@link Observation.Context}. This check avoids unnecessary
+     * {@link Observation.Context} creation, which is why it takes a {@link Supplier} for
+     * the context rather than the context directly. If the observation is not enabled
+     * (see
+     * {@link ObservationRegistry.ObservationConfig#observationPredicate(ObservationPredicate)
+     * ObservationConfig#observationPredicate}), a no-op observation will also be
+     * returned.
+     * @param name name of the observation
+     * @param contextSupplier mutable context supplier
+     * @param level observation level
+     * @param registry observation registry
+     * @return started observation
+     * @since 1.13.0
+     */
+    static <T extends Context> Observation start(String name, Supplier<T> contextSupplier, ObservationLevel level,
+            @Nullable ObservationRegistry registry) {
+        return createNotStarted(name, contextSupplier, level, registry).start();
+    }
+
+    /**
+     * Creates but <b>does not start</b> an {@link Observation}. Remember to call
+     * {@link Observation#start()} when you want the measurements to start. When no
+     * registry is passed or observation is not applicable will return a no-op
+     * observation.
+     * @param name name of the observation
+     * @param level observation level
+     * @param registry observation registry
+     * @return created but not started observation
+     * @since 1.13.0
+     */
+    static Observation createNotStarted(String name, ObservationLevel level, @Nullable ObservationRegistry registry) {
+        return createNotStarted(name, Context::new, level, registry);
+    }
+
     /**
      * Creates but <b>does not start</b> an {@link Observation}. Remember to call
      * {@link Observation#start()} when you want the measurements to start. When no
@@ -122,13 +176,38 @@ static Observation createNotStarted(String name, @Nullable ObservationRegistry r
      */
     static <T extends Context> Observation createNotStarted(String name, Supplier<T> contextSupplier,
             @Nullable ObservationRegistry registry) {
+        return createNotStarted(name, contextSupplier, null, registry);
+    }
+
+    /**
+     * Creates but <b>does not start</b> an {@link Observation}. Remember to call
+     * {@link Observation#start()} when you want the measurements to start. When the
+     * {@link ObservationRegistry} is null or the no-op registry, this fast returns a
+     * no-op {@link Observation} and skips the creation of the
+     * {@link Observation.Context}. This check avoids unnecessary
+     * {@link Observation.Context} creation, which is why it takes a {@link Supplier} for
+     * the context rather than the context directly. If the observation is not enabled
+     * (see
+     * {@link ObservationRegistry.ObservationConfig#observationPredicate(ObservationPredicate)
+     * ObservationConfig#observationPredicate}), a no-op observation will also be
+     * returned.
+     * @param name name of the observation
+     * @param contextSupplier supplier for mutable context
+     * @param level observation level
+     * @param registry observation registry
+     * @return created but not started observation
+     * @since 1.13.0
+     */
+    static <T extends Context> Observation createNotStarted(String name, Supplier<T> contextSupplier,
+            @Nullable ObservationLevel level, @Nullable ObservationRegistry registry) {
         if (registry == null || registry.isNoop()) {
             return NOOP;
         }
         Context context = contextSupplier.get();
         context.setParentFromCurrentObservation(registry);
+        context.setLevel(level != null ? level : null);
         if (!registry.observationConfig().isObservationEnabled(name, context)) {
-            return NOOP;
+            return new PassthroughNoopObservation(context.getParentObservation());
         }
         return new SimpleObservation(name, registry, context);
     }
@@ -178,7 +257,7 @@ static <T extends Context> Observation createNotStarted(@Nullable ObservationCon
             convention = registry.observationConfig().getObservationConvention(context, defaultConvention);
         }
         if (!registry.observationConfig().isObservationEnabled(convention.getName(), context)) {
-            return NOOP;
+            return new PassthroughNoopObservation(context.getParentObservation());
         }
         return new SimpleObservation(convention, registry, context);
     }
@@ -316,7 +395,7 @@ static <T extends Context> Observation createNotStarted(ObservationConvention<T>
         T context = contextSupplier.get();
         context.setParentFromCurrentObservation(registry);
         if (!registry.observationConfig().isObservationEnabled(observationConvention.getName(), context)) {
-            return NOOP;
+            return new PassthroughNoopObservation(context.getParentObservation());
         }
         return new SimpleObservation(observationConvention, registry, context);
     }
@@ -417,7 +496,7 @@ default Observation highCardinalityKeyValues(KeyValues keyValues) {
      * @return {@code true} when this is a no-op observation
      */
     default boolean isNoop() {
-        return this == NOOP;
+        return this == NOOP || this instanceof NoopObservation;
     }
 
     /**
@@ -923,7 +1002,13 @@ class Context implements ContextView {
         private Throwable error;
 
         @Nullable
-        private ObservationView parentObservation;
+        private ObservationView thisObservation;
+
+        @Nullable
+        private ObservationView parentObservationView;
+
+        @Nullable
+        private ObservationLevel level;
 
         private final Map<String, KeyValue> lowCardinalityKeyValues = new LinkedHashMap<>();
 
@@ -970,15 +1055,15 @@ public void setContextualName(@Nullable String contextualName) {
          */
         @Nullable
         public ObservationView getParentObservation() {
-            return parentObservation;
+            return parentObservationView;
         }
 
         /**
          * Sets the parent {@link ObservationView}.
          * @param parentObservation parent observation to set
          */
         public void setParentObservation(@Nullable ObservationView parentObservation) {
-            this.parentObservation = parentObservation;
+            this.parentObservationView = parentObservation;
         }
 
         /**
@@ -987,7 +1072,7 @@ public void setParentObservation(@Nullable ObservationView parentObservation) {
          * @param registry the {@link ObservationRegistry} in using
          */
         void setParentFromCurrentObservation(ObservationRegistry registry) {
-            if (this.parentObservation == null) {
+            if (this.parentObservationView == null) {
                 Observation currentObservation = registry.getCurrentObservation();
                 if (currentObservation != null) {
                     setParentObservation(currentObservation);
@@ -1232,12 +1317,21 @@ public KeyValues getAllKeyValues() {
             return getLowCardinalityKeyValues().and(getHighCardinalityKeyValues());
         }
 
+        @Nullable
+        public ObservationLevel getLevel() {
+            return level;
+        }
+
+        void setLevel(ObservationLevel level) {
+            this.level = level;
+        }
+
         @Override
         public String toString() {
             return "name='" + name + '\'' + ", contextualName='" + contextualName + '\'' + ", error='" + error + '\''
                     + ", lowCardinalityKeyValues=" + toString(getLowCardinalityKeyValues())
                     + ", highCardinalityKeyValues=" + toString(getHighCardinalityKeyValues()) + ", map=" + toString(map)
-                    + ", parentObservation=" + parentObservation;
+                    + ", parentObservation=" + parentObservationView + ", observationLevel=" + level;
         }
 
         private String toString(KeyValues keyValues) {
@@ -1452,6 +1546,14 @@ default <T> T getOrDefault(Object key, Supplier<T> defaultObjectSupplier) {
         @NonNull
         KeyValues getAllKeyValues();
 
+        /**
+         * Returns the observation level.
+         * @return observation level
+         */
+        default Level getObservationLevel() {
+            return Level.ALL;
+        }
+
     }
 
     /**
@@ -1487,4 +1589,103 @@ interface CheckedFunction<T, R, E extends Throwable> {
 
     }
 
+    /**
+     * Mapping of {@link Level} to {@link Class}.
+     *
+     * @author Marcin Grzejszczak
+     * @since 1.13.0
+     */
+    class ObservationLevel {
+
+        private final Level level;
+
+        private final Class<?> clazz;
+
+        public ObservationLevel(Level level, Class<?> clazz) {
+            this.level = level;
+            this.clazz = clazz;
+        }
+
+        public Level getLevel() {
+            return level;
+        }
+
+        public Class<?> getClazz() {
+            return clazz;
+        }
+
+        /**
+         * Sets {@link Level#ALL} for observation of the given classs.
+         * @param clazz class to observe
+         * @return observation level
+         */
+        public static ObservationLevel all(Class<?> clazz) {
+            return new ObservationLevel(Level.ALL, clazz);
+        }
+
+        /**
+         * Sets {@link Level#TRACE} for observation of the given classs.
+         * @param clazz class to observe
+         * @return observation level
+         */
+        public static ObservationLevel trace(Class<?> clazz) {
+            return new ObservationLevel(Level.TRACE, clazz);
+        }
+
+        /**
+         * Sets {@link Level#DEBUG} for observation of the given classs.
+         * @param clazz class to observe
+         * @return observation level
+         */
+        public static ObservationLevel debug(Class<?> clazz) {
+            return new ObservationLevel(Level.DEBUG, clazz);
+        }
+
+        /**
+         * Sets {@link Level#INFO} for observation of the given classs.
+         * @param clazz class to observe
+         * @return observation level
+         */
+        public static ObservationLevel info(Class<?> clazz) {
+            return new ObservationLevel(Level.INFO, clazz);
+        }
+
+        /**
+         * Sets {@link Level#WARN} for observation of the given classs.
+         * @param clazz class to observe
+         * @return observation level
+         */
+        public static ObservationLevel warn(Class<?> clazz) {
+            return new ObservationLevel(Level.WARN, clazz);
+        }
+
+        /**
+         * Sets {@link Level#ERROR} for observation of the given classs.
+         * @param clazz class to observe
+         * @return observation level
+         */
+        public static ObservationLevel error(Class<?> clazz) {
+            return new ObservationLevel(Level.ERROR, clazz);
+        }
+
+        /**
+         * Sets {@link Level#FATAL} for observation of the given classs.
+         * @param clazz class to observe
+         * @return observation level
+         */
+        public static ObservationLevel fatal(Class<?> clazz) {
+            return new ObservationLevel(Level.FATAL, clazz);
+        }
+
+        /**
+         * Sets {@link Level#OFF} for observation of the given classs.
+         * @param clazz class to observe
+         * @return observation level
+         */
+        public static ObservationLevel off(Class<?> clazz) {
+            return new ObservationLevel(Level.OFF, clazz);
+        }
+
+    }
+
 }