Support Guzzle 6 and Guzzle 7

Author: LKaemmerling

composer.json

@@ -4,7 +4,7 @@
   "type": "library",
   "require": {
     "php": "^7.2",
-    "guzzlehttp/guzzle": "^6.0|^7.0",
+    "guzzlehttp/guzzle": "^6.3|^7.0",
     "lkaemmerling/prometheus_client_php": "^1.0|^2.0"
   },
   "require-dev": {