Allow using v2.0.0 of prometheus client

Author: LKaemmerling

README.md

@@ -39,7 +39,7 @@ Also look at the [examples](examples).
 
 ## Black box testing
 
-Just start the pushgateway by using docker-compose
+Just start the PushGateway by using docker-compose
 ```
 docker-compose up
 ```

composer.json

@@ -5,7 +5,7 @@
   "require": {
     "php": "^7.2",
     "guzzlehttp/guzzle": "^7.0",
-    "lkaemmerling/prometheus_client_php": "^1.0"
+    "lkaemmerling/prometheus_client_php": "^1.0|^2.0"
   },
   "require-dev": {
     "phpunit/phpunit": "^8.4",