Better metrics (forwarders)

[worldtiki]

This is an improvement to an existing feature.

We started capturing and reporting metrics a while back, and since then we added support for graphite and prometheus systems.
However most of the metrics we capture focus only on the ingestion side of Pitchfork. We don't have metrics that measure the performance or health of each of the individual forwarders.

[worldtiki]

Example of one of the forwarders where we don't have a metrics counter and should:
https://github.com/HotelsDotCom/pitchfork/blob/master/src/main/java/com/hotels/service/tracing/zipkintohaystack/forwarders/zipkin/http/ZipkinForwarder.java#L79