Add Support for storing HTTP Headers (Req & Resp) for gRPC calls

[jpopadak]

Is your feature request related to a problem? Please describe.
There does not seem to be any documentation on how view headers from a gRPC request.

For example, the Host gets set for apmhttp (See #280) but not for apmgrpc. Why not, when grpc is over an HTTP2 connection?

Describe the solution you'd like
I would love if it could store the same request headers and the same response headers as apmhttp modules does. Thus, its easy to see standard headers from the apmgrpc interceptors. Ideality, it should happen in getIncomingMetadataTraceContext since it already pulls the Request Metadata.

Describe alternatives you've considered
Alternatives include creating our own interceptor that runs after the apmgrpc interceptor. It adds the headers to the parent span.

Additional context

[axw]

Thanks for opening this @jpopadak. I agree, it would be very sensible to include the underlying HTTP-related context.