Extract context clearing logic for ActiveSupport events

[marcotc]

When gems that utilize thread pools are instrumented it is very important to ensure that the thread-bound Context does not leak information between executions.

We have existing logic to clear the Context for a few integrations that instrument using ActiveSupport events.

This PR extracts that logic to a central place, adds a health metric to count how many times the context is leaking and cleans up testing for integrations affected.

[marcotc]

Now that #824 has been merged, I'm rebasing this on master instead.

[marcotc]

Rebased, ready for review again.