This adds a gauge exposing the configured number of threads we expect
to be running and using the database called
`gitlab_ruby_threads_concurrency_total`.

It also adds `gitlab_ruby_threads_total`.

This gauge has a `thread_name` label used to identify the kind of
thread. For unnamed threads the label will contain `unnamed`.

To identify sidekiq worker threads, that before had no name, we set it
in the `Gitlab::SidekiqMiddleware::Metrics` server middleware. So we
can distinguish it from other running threads.

The `uses_db_connection` label indicates if the thread has at some
point connected to the database. We use the same subscriber as we do
for the query counts to flip this value on the thread.