Prometheus metrics
Closes https://gitlab.com/gitlab-org/gitlab-workhorse/issues/61
This adds two metrics to begin with: total requests served, partitioned by method/code, and the number of currently active requests.
Things to check:
-
are the names of the metrics OK -
is the access mechanism (unauthenticated HTTP at /metrics ) OK -
check licenses of new dependencies: a mix of MIT, Apache 2.0, and BSD 3-clause
Note to reviewers: only the first four files in the GitLab diff are outside vendor/. No need to scroll through the other 87 files.