Add instructions in CONTRIBUTING.md on how to contribute new metrics (o…

…pen-telemetry#11043)

* Add Instructions For New Metrics

Added detailed steps to add new metrics for receiver.

* Apply suggestions from code review

Co-authored-by: Alex Boten <[email protected]>

* Add note that these change only applies to existing receivers

* Update CONTRIBUTING.md

Co-authored-by: Alex Boten <[email protected]>

* add links to  what we already have

Co-authored-by: Alex Boten <[email protected]>

CONTRIBUTING.md

@@ -78,6 +78,18 @@ The following GitHub users are the currently available sponsors, either by being
 * [@djaglowski](https://github.com/djaglowski)
 
 Whenever a sponsor is picked from the top of this list, please move them to the bottom.
+
+## Adding metrics to existing receivers
+Following these steps for contributing additional metrics to existing receivers.
+ - Read instructions [here](https://github.com/open-telemetry/opentelemetry-collector/blob/main/CONTRIBUTING.md#fork) on how to 
+ fork, build and create PRs. The only difference is to change repository name from `opentelemetry-collector` to `opentelemetry-collector-contrib`
+ - Edit `metadata.yaml` of your metrics receiver to add new metrics, e.g.: `redisreceiver/metadata.yaml`
+ - To generate new metrics on top of this updated YAML file.
+ - Run `cd receiver/redisreceiver`
+ - Run `go generate ./...`
+- Review the changed files and merge the changes into your forked repo.
+- Create PR from Github web console following the instructions above.
+
 ## General Recommendations
 Below are some recommendations that apply to typical components. These are not rigid rules and there are exceptions but
 in general try to follow them.