Add support for Akka HTTP flow-based (super-pool) client requests

[wuservices]

This request is for flow-based requests to be instrumented. Support for this was removed in 681420a and #899, but the old implementation may not have worked.

Currently it looks like the agent only supports single client requests. Akka HTTP also offers a flow-based variant, which is presented by the Http().superPool(...) method and routes requests to a cached connection pool.

While this would be helpful for all Akka HTTP users using the flow-based variant, I'm using elastic4s for Elasticsearch from Scala with its Akka HTTP client. I was wondering why I couldn't see the HTTP requests, but I'm thinking it's due to lack of support for a super-pool flow-based client request used in elastic4s.