Implement tag propagation for HTTP

[odeke-em]

Issue to implement HTTP tag propagation as its lack of came up in the interop testing.

[rakyll]

OpenCensus doesn't provide a common format for tag propagation over HTTP at this point, we need to discuss this issue at https://github.com/census-instrumentation/opencensus-specs.

[odeke-em]

Thanks for confirming that, I wanted to file one last week but I didn't see the gRPC tag propagation spec information either, yet we have tag propagation already for gRPC which are sent as Metadata trailers.