AsyncHTTPClient should export metrics

[fabianfett]

Developers want to monitor metrics of outgoing http requests to identify potential causes of issues with dashboards.

While it is possible to generate some metrics by implementing a custom HTTPClientResponseDelegate today, this is quite a burden. Further there are some metrics AsyncHTTPClient users don't have access to. For example:

• Number of active http/1.1 requests to a host
• Number of requests in the queue to a host

The metrics should be exported to SwiftMetrics directly or with a delegate protocol call comparable to URLSession:task:didFinishCollectingMetrics: exporting NSURLSessionTaskMetrics. If the latter approach is chosen, there should also be a callback/delegate to observe the connection pool size and requests queued per ConnectionPool.Key.

[weissi]

• how many Waiters do we have
• how many connections do we have open
• how many connections are parked in the pool