Fix percentile metrics rotating too often

Before this change percentile metrics rotation interval was set to `expiry / bufferLength`. This was different from other metrics like `_max` where the interval is set to `expiry`. At the same time documentation of the `expiry` parameter says clearly that it is used as rotation interval.

This inconsistent behavior was confusing for users because some metrics expired faster than the others. What's more it caused some requests to be ignored in percentile metrics if `expiry` (also called `step`) was set to the same duration as scrapping interval (e.g. 1 minute) and scrapping occurred not long after buffer rotation. In case of the default config where `bufferLength` is 3 it could result in up to 33% requests being ignored by percentile metrics.

Fix this inconsistency by changing buffer rotation interval for percentiles to `expiry`.

Fixes micrometer-metrics#3298

Author: rafalh

Diff for: micrometer-core/src/main/java/io/micrometer/core/instrument/distribution/AbstractTimeWindowHistogram.java

@@ -74,10 +74,10 @@ abstract class AbstractTimeWindowHistogram<T, U> implements Histogram {
 
         ringBuffer = (T[]) Array.newInstance(bucketType, ageBuckets);
 
-        durationBetweenRotatesMillis = distributionStatisticConfig.getExpiry().toMillis() / ageBuckets;
+        durationBetweenRotatesMillis = distributionStatisticConfig.getExpiry().toMillis();
         if (durationBetweenRotatesMillis <= 0) {
             rejectHistogramConfig("expiry (" + distributionStatisticConfig.getExpiry().toMillis()
-                    + "ms) / bufferLength (" + ageBuckets + ") must be greater than 0.");
+                    + "ms) must be greater than 0.");
         }
 
         currentBucket = 0;

Diff for: micrometer-core/src/test/java/io/micrometer/core/instrument/distribution/TimeWindowPercentileHistogramTest.java

@@ -162,7 +162,7 @@ private boolean percentileValueIsApproximately(ValueAtPercentile vp, double perc
     void timeBasedSlidingWindow() {
         final DistributionStatisticConfig config = DistributionStatisticConfig.builder()
             .percentiles(0.0, 0.5, 0.75, 0.9, 0.99, 0.999, 1.0)
-            .expiry(Duration.ofSeconds(4))
+            .expiry(Duration.ofSeconds(1))
             .bufferLength(4)
             .build()
             .merge(DistributionStatisticConfig.DEFAULT);