doc: Add documentation on CLI metrics (#224)

* doc: Document CLI metrics

* doc: Add examples

docs/metrics.md

@@ -0,0 +1,43 @@
+## Metrics
+
+Meroxa CLI currently uses [Cased](https://cased.com/) to track user behaviour with the purpose of making informed decisions around sunsetting commands, introducing aliases when we notice our customers  _claim_ for one, and a long etc.
+
+These metrics are pushed automatically for any command that's nested under the root `meroxa` command, and its code configuration lives in [/cmd/meroxa/global/metrics](/cmd/meroxa/global/metrics.go).
+
+We don't log flag values, and only their names so no sensitive information is actually being tracked.
+
+### Configuration
+
+CLI uses some environment variables you can modify in your configuration file (you can check the one you're using by doing `meroxa config`) which will have different effects:
+
+- [`CASED_PUBLISH_KEY`](/cmd/meroxa/global/metrics.go#L37) - set the publish key you'd like to use. When developing locally, you should use the API Key with the `publish` type on a testing audit trail in Cased.com. Alternatively, if you don't set this API Key, you would test publishing through a proxy (e.g.: Staging API for example).
+- [`CASED_DEBUG`](/cmd/meroxa/global/metrics.go#L46-L48) - set it to `true` if you want to see confirmation on whether events are actually published.
+
+Example:
+
+```shell
+$ .m resources ls
+ID         NAME           TYPE     ENVIRONMENT                                         URL                                         TUNNEL   STATE 
+====== ================= ========== ============= ================================================================================= ======== =======
+... 
+
+[Cased] 2021/12/07 18:15:56 Successfully published audit event.
+[Cased] 2021/12/07 18:15:56 Published all audit events in buffer.
+
+``` 
+
+- [`PUBLISH_METRICS`](/cmd/meroxa/global/metrics.go#L42-L44) - set this to `false` if you don't want to publish any metric to cased. When modifying how these metrics could be sent to Cased, you can set that to `stdout` to see the event in standard out.
+
+Example (with `PUBLISH_METRICS=stdout`):
+
+```shell 
+$ .m resources ls
+  ID         NAME           TYPE     ENVIRONMENT                                         URL                                         TUNNEL   STATE 
+====== ================= ========== ============= ================================================================================= ======== =======
+... 
+
+Event: {"action":"meroxa.resources.list","actor":"[email protected]","actor_uuid":"b9d73867-f7c8-49a0-a7c5-c655b7f1258a","command":{"alias":"ls"},"timestamp":"2021-12-07T17:14:16.917903Z","user_agent":"meroxa/dev darwin/amd64"}
+```
+
+In this last case, we wouldn't be emitting metrics to cased at all, so things such as `CASED_DEBUG` and `CASED_PUBLISH_KEY` wouldn't really matter.
+