AsyncHTTPClient should export metrics #342
Description
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.