Add metrics for OkHttp ConnectionPool

[benhubert]

Hi! I think it would be helpful to have metrics for OkHttp's ConnectionPool usage.

I already implemented this for us and created a PR. I'm uncertain about some implementation details, see my comments in the PR for details.

[jkschneider]

@benhubert Would you like to open a PR with this implementation? Seems like you've got it working...

[benhubert]

Well, that was my intention, but seems that I forgot to open the PR. Thanks for your hint.
Now it's ready for review: #2066