Stop generating metrics.md and direct to Glean Dictionary instead

[wlach]

Now that the Glean Dictionary (https://dictionary.telemetry.mozilla.org)
has hit production, our (= Mozilla Data's) guidance is to move away from generating
metrics.md files. The Glean Dictionary is a better solution for this task
for a couple of reasons:

1. The Glean Dictionary contains a complete reference of all metrics
   and pings sent by an application (not just those defined in the
   application itself, as is the case with the markdown documentation)
2. The Glean Dictionary provides additional context and links beyond
   what is provided by the documentation (for example, information on
   how to access metric information via BigQuery or GLAM) as well as
   annotations provided by data scientists in the course of analyzing data
   (see, for example, the commentary section for:
   https://dictionary.telemetry.mozilla.org/apps/fenix/metrics/metrics_search_count).

This should also reduce some of the churn/burden on people when
they submit PRs, as there is no longer any need to update metrics.md
as a part of them.

Pull Request checklist

• Tests: This PR includes thorough tests or an explanation of why it does not
• Screenshots: This PR includes screenshots or GIFs of the changes made or an explanation of why it does not
• Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features. In addition, it includes a screenshot of a successful accessibility scan to ensure no new defects are added to the product.

To download an APK when reviewing a PR:

1. click on Show All Checks,
2. click Details next to "Taskcluster (pull_request)" after it appears and then finishes with a green checkmark,
3. click on the "Fenix - assemble" task, then click "Run Artifacts".
4. the APK links should be on the left side of the screen, named for each CPU architecture

[firefoxci-taskcluster]

No Taskcluster jobs started for this pull request

The `allowPullRequests` configuration for this repository (in `.taskcluster.yml` on the
default branch) does not allow starting tasks for this pull request.

[wlach]

Note that the data review guidelines have been updated to allow/reflect the fact that the Glean Dictionary is the source of truth:

• Update suggested wording for specifying where Glean metrics are documented mozilla/data-review#27
• New Glean Dictionary URL mozilla/data-review#29

[gabrielluong]

@wlach I assume we can also do this for https://github.com/mozilla-mobile/android-components?

[wlach]

@wlach I assume we can also do this for https://github.com/mozilla-mobile/android-components?

We don't currently have any view of libraries in the glean dictionary seperate from the applications that include them, so it might be best to keep the metrics.md files for those around for now, just so there is a place to look for all the metrics that correspond to a particular component. I filed mozilla/glean-dictionary#572 to look into this issue though.