Metrics interface and Prometheus implementation (#1929)

* Metrics interface

* Prometheus implementation

* NoOp implementation

Co-authored-by: chris <chris@Profanity.local>

metrics/prometheus/README.md

@@ -0,0 +1,26 @@
+Prometheus
+==========
+
+A Prometheus "pull" based implementation of the metrics Reporter interface.
+
+
+Capabilities
+------------
+
+* Go runtime metrics are handled natively by the Prometheus client library (CPU / MEM / GC / GoRoutines etc).
+* User-defined metrics are registered in the Prometheus client dynamically (they must be pre-registered, hence all of the faffing around in metric_family.go).
+* The metrics are made available on a Prometheus-compatible HTTP endpoint, which can be scraped at any time. This means that the user can very easily access stats even running locally as a standalone binary.
+* Requires a micro.Server parameter (from which it gathers the service name and version). These are included as tags with every metric.
+
+
+Usage
+-----
+
+```golang
+    prometheusReporter := metrics.New(server)
+    tags := metrics.Tags{"greeter": "Janos"}
+    err := prometheusReporter.Count("hellos", 1, tags)
+    if err != nil {
+        fmt.Printf("Error setting a Count metric: %v", err)
+    }
+```