Add documentation for Prometheus trace metrics

What does this MR do and why?

Related to #342719 (closed)

We noticed in !72001 (merged) that the metrics from https://gitlab.com/gitlab-org/gitlab/-/blob/059270460a306c26b5390dacc1a8d34b5c2a90e6/lib/gitlab/ci/trace/metrics.rb#L54-81 are not included in our docs.

Describe in detail what your merge request does and why.

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