added metrics for OkHttp connection pool

[benhubert]

See #1875

[jkschneider]

What I don't like about it, that the idle value is a subset of total, so accessing okhttp.pool.connection.count without any tag won't give you the overall sum. Instead it will give you the overall sum + the idle connections, which is not a useful value.

I think I've got a solution for the synchronization problem. Give me a few moments.

[jkschneider]

Fixes #1875

[jkschneider]

See if this makes sense @benhubert. We just hold total and idle together in a wrapper and use a CountDownLatch to refresh the stats whenever both gauges have been published and one tries to access the value on the next publish interval.

The stats wrapper is held in a ThreadLocal so if multiple metrics publishings happen simultaneously (e.g. multiple overlapping Prometheus scrapes), then the synchronization is only for the two gauges being published as part of the same metrics publishing operation.