[vcs] add vcs.change.time_to_merge metric

[adrielp]

Changes

Adds the vcs.change.time_to_merge metric from the GitHub Receiver. It didn't make as part of a previous pull request.

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• schema-next.yaml updated with changes to existing conventions.

[christophe-kamphaus-jemmic]

The vcs.change.time_to_merge metric was removed in the previous PR following this discussion: #1383 (comment)
I thought that vcs.change.state attribute on the vcs.change.duration metric was sufficient to distinguish them, but it makes sense to have time_to_merge separate.

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 7 days.

[christophe-kamphaus-jemmic]

Let's add the vcs.change.time_to_merge metric in the markdown. cf #1685 (comment)