Now both HTTP/1.1 and HTTP/2 follow the documented format.
HTTP/1.1 was including an extra element containing the
StreamID before, which was unnecessary because it is also
given as argument to the callback.
HTTP/2 early_error will now include headers in its PartialReq.
This allows giving custom metadata to the metrics stream handler.
This can be useful to for example provide the name of the
module handling the request which is only known after routing.
But any user data is allowed.
When called multiple times the user data maps are merged.
It collects metrics and passes them to a configurable callback
once the stream terminates. It will be documented in a future
release. More tests incoming.
