Make Gitlab::GitalyClient emit events for monitoring, rather than manually tracking

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The following discussion from gitlab-ce!17564 should be addressed:

• @smcgivern started a discussion: (+1 comment)

  Given the amount of complexity woven through this class now, I wonder if we should consider doing something like http://api.rubyonrails.org/classes/ActiveSupport/Notifications.html instead?

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Gitlab::GitalyClient has these (at least) in it:

1. Gitlab::Metrics
2. N+1 detection
3. Call details, for the performance bar

At least the first and third of these could benefit from a solution similar to ActiveSupport::Notifications. These are already used in the same cases (performance bar and histograms) for SQL, by subscribing to sql.active_record and friends.