This change adds application/x-protobuf support to the existing
grpc-gateway mux in the OTLP receiver.

See: https://github.com/open-telemetry/oteps/blob/master/text/0099-otlp-http.md

What currently works:
* The receiver will correctly process HTTP requests with the
  Content-Type application/x-protobuf.
* The receiver will respond with Content-Type application/x-protobuf.
* The receiver will respond with status code HTTP 200 OK on success.
What doesn't work yet:
* The receiver cannot handle gzip-encoded requests (Content-Encoding
  gzip).
* The receiver will not gzip-encode responses to requests with
  Accept-Encoding: gzip.


Updates open-telemetry#881

runtime.ProtoMarshaller is now embedded in xProtoBufMarshaler so that only ContentType() needs to be implemented.