Generate metrics.md for GeckoView updates?

[pocmo]

The old script always built the engine component(s), which may generate a new metrics.md if telemetry probes were added, changed or removed in GeckoView. With that we were able to add it to the pull request that updated the version:
https://github.com/mozilla-mobile/android-components/blob/master/automation/taskcluster/action/update_geckoview.sh#L62-L66

Now we only bump the version, which means the file gets generated locally for every engineer that builds that component. And then someone needs to check it in. I wonder if we can get relbot to generate this file too? 🤔