Metrics interface and Prometheus implementation (#1929)

* Metrics interface

* Prometheus implementation

* NoOp implementation

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

metrics/README.md

@@ -0,0 +1,22 @@
+metrics
+=======
+
+The metrics package provides a simple metrics "Reporter" interface which allows the user to submit counters, gauges and timings (along with key/value tags).
+
+Implementations
+---------------
+
+* Prometheus (pull): will be first
+* Prometheus (push): certainly achievable
+* InfluxDB: could quite easily be done
+* Telegraf: almost identical to the InfluxDB implementation
+* Micro: Could we provide metrics over Micro's server interface?
+
+
+Todo
+----
+
+* Include a handler middleware which uses the Reporter interface to generate per-request level metrics
+    - Throughput
+    - Errors
+    - Duration