Enable prometheus metrics for Focalboard to enable observability (#414)

As an SRE team we would like to expose standard metrics and grouped by version
of the application. Right now will expose only metrics related to Go but instrumentor
should be used in other parts of the codebase so we can track other metrics, eg.
number of tasks, boards, users etc. Similar as we do in MM.

It will run in port `localhost:9092` and it is a new config `prometheus_address`.

Also in the commit we introduced, `group.Add` which helps us to handle gracefully
errors for goroutines. It's a good practice and there are couple of articles by
Golang for this.

docker/config.json

@@ -8,6 +8,7 @@
     "webpath": "./pack",
     "filespath": "./files",
     "telemetry": true,
+    "prometheus_address": ":9092",
     "session_expire_time": 2592000,
     "session_refresh_time": 18000,
     "localOnly": false,