gRPC tag propagation

[c24t]

Like the span context, we need to be able to propagate the TagMap along with the request. We need to do this for both gRPC and HTTP.

#209 added binary serialization for the tag map and #257 added the option to set and read the tag map from the execution context. The last step is to update the gRPC client interceptor to read the tag map from the gRPC metadata and insert it into the OC execution context and update the gRPC server interceptor to attach the tag map to gRPC responses as metadata. This is similar to the way the gRPC client and server handle the trace header now.

See also binary serialization and gRPC propagation in the java client and go client.