OpenCensus Agent Reporter

This reporter exports spans to the OpenCensus Agent in the standard protobuf format over grpc.

To use, add opencensus_service dependency as a runtime application (in rebar3 this means add to the applications list of .app.src) and set as the reporter in the opencensus configuration:

{opencensus, [
    {reporter, {oc_reporter_service, []}
...]}