Added documentation for grabbit metrics (#117)

* added initial documentation for grabbit metrics

*  including metrics section in readme.md

README.md

@@ -21,6 +21,7 @@ A lightweight transactional message bus on top of RabbitMQ supporting:
 5) [Reliable messaging](https://github.com/wework/grabbit/blob/master/docs/OUTBOX.md) and local service transactivity via Transaction Outbox pattern
 6) Deadlettering
 7) [Structured logging](https://github.com/wework/grabbit/blob/master/docs/LOGGING.md)
+8) Reporting [metrics](https://github.com/wework/grabbit/blob/master/docs/METRICS.md) via Prometheus
 
 Planned:
 

docs/METRICS.md

@@ -0,0 +1,9 @@
+# Metrics
+
+grabbit exposes and reports operational the following metrics to Prometheus
+
+|  Namespace    | Subsystem     | Name                              | Description                                                                 |
+| ------------- | ------------- | ----------------------------------| --------------------------------------------------------------------------- |
+| grabbit       | handler       | [name of message handler]_result  | records and counts each succesfull or failed execution of a message handler |
+| grabbit       | handler       | [name of message handler]_latency | records the execution time of each handler                                  |
+| grabbit       | messages      | rejected_messages                 | increments each time a message gets rejected                                |