Support BraveRpcService

[jrhee17]

Motivation

For high-traffic services, it may be unfeasible to add tracing for all requests due to the large volume.
It is a common practice to sample traces for specific requests (e.g. by user id) instead of random sampling.

Currently, BraveService already provides a way to do this via either 1) Tracing.newBuilder().sampler(...) or 2) HttpTracing.newBuilder(...).serverSampler().
However, sometimes users would like to sample based on a parameter parsed by rpc requests.

Because BraveService determines whether a trace should be sampled before the request is parsed into a rpc request, it is difficult for users to sample based on RPC requests.

Proposal

I propose that a BraveRpcService is introduced. By doing so, users may sample traces based on RPC parameters more easily.
By doing so, users will be able to sample traces based on ServiceRequestContext#rpcRequest

i.e.

RpcTracing.newBuilder(tracing)
          .serverSampler(arg -> {
              final ServiceRequestContext ctx = (ServiceRequestContext) arg.unwrap();
              final RpcRequest rpcRequest = ctx.rpcRequest();
              if (rpcRequest != null && "...".equals(rpcRequest.serviceName())) {
                  return true;
              }
              return null;
          });

One caveat is that BraveRpcService sampler won't have an effect if a BraveService is already in the decorator chain and was processed.
The additional span containing RPC specific information would still probably be useful though.

[minwoox]

Sounds good. 👍