Add dangerbot reminder to update metrics docs

Problem to solve

When a new metric is added to GitLab, we tend to forget to update the documentation as well.

The DangerBot can have a reminder to update the documentation, similar to how it reminds to update the schema.rb file if there is a migration in the MR.

Intended users

• Sasha (Software Developer)

Further details

Proposal

If the MR contains code like the following, add a message to the DangerBot comment that reminds to update documentation.

1. Gitlab::Metrics.counter
2. Gitlab::Metrics.gauge
3. Gitlab::Metrics.summary
4. Gitlab::Metrics.histogram

Documentation link: https://docs.gitlab.com/ee/administration/monitoring/prometheus/gitlab_metrics.html#metrics-available

Message:

If you have added new GitLab metrics, please make sure to also update the documentation: doc/administration/monitoring/prometheus/gitlab_metrics.md

Permissions and Security

Documentation

Update the Danger documentation: doc/development/dangerbot.md

Availability & Testing

What does success look like, and how can we measure that?

MRs that add GitLab metrics should have a reminder to update documentation in the DangerBot comment.

What is the type of buyer?

Links / references