docs(metrics): available metrics, metrics enablement (runatlantis#3171)

* docs: documentation for available metrics/stats

* docs: refactored as per comments

* docs: update metrics description

* docs: add empty lines between blocks

* docs: add note section

* docs: add missing dot

* Apply suggestions from code review

---------

Co-authored-by: Pardeep Bhatt <pardeep.bhatt@gojek.com>
Co-authored-by: nitrocode <7775707+nitrocode@users.noreply.github.com>

runatlantis.io/docs/stats.md

@@ -9,3 +9,59 @@ Currently Statsd and Prometheus is supported. See configuration below for detail
 ## Configuration
 
 Metrics are configured through the [Server Side Config](server-side-repo-config.html#metrics).
+
+## Available Metrics
+
+Assuming metrics are exposed from the endpoint `/metrics` from the [metrics](server-side-repo-config.html#metrics) server side config e.g.
+
+
+```yaml
+metrics:
+  prometheus:
+    endpoint: "/metrics"
+```
+
+
+To see all the metrics exposed from atlantis service, make a GET request to the `/metrics` endpoint.
+
+
+```bash
+curl localhost:4141/metrics
+# HELP atlantis_cmd_autoplan_builder_execution_error atlantis_cmd_autoplan_builder_execution_error counter
+# TYPE atlantis_cmd_autoplan_builder_execution_error counter
+atlantis_cmd_autoplan_builder_execution_error 0
+# HELP atlantis_cmd_autoplan_builder_execution_success atlantis_cmd_autoplan_builder_execution_success counter
+# TYPE atlantis_cmd_autoplan_builder_execution_success counter
+atlantis_cmd_autoplan_builder_execution_success 10
+# HELP atlantis_cmd_autoplan_builder_execution_time atlantis_cmd_autoplan_builder_execution_time summary
+# TYPE atlantis_cmd_autoplan_builder_execution_time summary
+atlantis_cmd_autoplan_builder_execution_time{quantile="0.5"} NaN
+atlantis_cmd_autoplan_builder_execution_time{quantile="0.75"} NaN
+atlantis_cmd_autoplan_builder_execution_time{quantile="0.95"} NaN
+atlantis_cmd_autoplan_builder_execution_time{quantile="0.99"} NaN
+atlantis_cmd_autoplan_builder_execution_time{quantile="0.999"} NaN
+atlantis_cmd_autoplan_builder_execution_time_sum 11.42403017
+atlantis_cmd_autoplan_builder_execution_time_count 10
+.....
+.....
+.....
+```
+
+
+::: tip NOTE
+The output shown above is trimmed, since with every new version release this metric set will need to be updated accordingly as there may be a case if some metrics are added/modified/deprecated, so the output shown above just gives a brief idea of how these metrics look like and rest can be explored.
+:::
+
+Important metrics to monitor are
+
+| Metric Name                                    | Metric Type                                                          | Purpose                                                                                                            |
+|------------------------------------------------|----------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------|
+| `atlantis_cmd_autoplan_execution_error`        | [counter](https://prometheus.io/docs/concepts/metric_types/#counter) | number of times when [autoplan](autoplanning.html#autoplanning) has thrown error. |
+| `atlantis_cmd_comment_plan_execution_error`    | [counter](https://prometheus.io/docs/concepts/metric_types/#counter) | number of times when on commenting `atlantis plan` has thrown error.      |
+| `atlantis_cmd_autoplan_execution_success`      | [counter](https://prometheus.io/docs/concepts/metric_types/#counter) | number of times when [autoplan](autoplanning.html#autoplanning) has run successfully. |
+| `atlantis_cmd_comment_apply_execution_error`   | [counter](https://prometheus.io/docs/concepts/metric_types/#counter) | number of times when on commenting `atlantis apply` has thrown error.     |
+| `atlantis_cmd_comment_apply_execution_success` | [counter](https://prometheus.io/docs/concepts/metric_types/#counter) | number of times when on commenting `atlantis apply` has run successfully. |
+
+::: tip NOTE
+There are plenty of additional metrics exposed by atlantis that are not described above.
+:::