Fix #660: Add the rate metric type

[mdboom]

Technically this is a backward-incompatible change. However, there is nothing in the wild sending a rate metric type yet, so I believe this is ok (and should be even when we get to the schema generator part of this). But please check my assumption, @frank and @acmiyaguchi .

Checklist for reviewer:

• Commits should reference a bug or github issue, if relevant (if a bug is referenced, the pull request should include the bug number in the title)
• If adding a new field, the field should have a description (see Bug 1652842 - Add a blocklist field so the environment schema works with pre- and post-bug-1647225 data #576 for an example)
• Scan the PR and verify that no changes (particularly to .circleci/config.yml) will cause environment variables (particularly credentials) to be exposed in test logs
• If the PR comes from a fork, trigger the integration CI test by pushing this revision as discussed in the README and review the report posted in the comments.

For glean changes:

• Update templates/include/glean/CHANGELOG.md

[jklukas]

Pulling down generated-schemas locally, I was able to confirm no rates or labeled_rates exist in the wild:

› find schemas -name "*.bq" | xargs grep '"rate"' | wc
       0       0       0

› find schemas -name "*.bq" | xargs grep '"labeled_rate"' | wc
       0       0       0

Compare this to searching for "counter":

› find schemas -name "*.bq" | xargs grep '"counter"' | wc
      76     228    6251

[fbertsch]

So, we are actually generating rate and labeled_rate cols if they appear in probe-info-service. However, I have confirmed that no apps have ever reported a rate or labeled_rate type, so we should be good to go.

Aside, are we implementing labeled_rate?

[mdboom]

Aside, are we implementing labeled_rate?

No, there are a bunch of labeled_X metric types from the original design doc that have been in here from the beginning that aren't implemented. They should probably be removed, but maybe as a follow-on bug?