ReportRateLimit Tracing Headers

[eguzki]

Following the introduction of the new CheckRateLimit gRPC method in #439, we have an opportunity to provide richer tracing to rate-limited clients.

service RateLimitService {
  // The report method modifies specific counters associated with the descriptors, incrementing each one by the amount specified in $hits_addend$
  rpc Report(envoy.service.ratelimit.v3.RateLimitRequest) returns (envoy.service.ratelimit.v3.RateLimitResponse); 
}

I propose that the tracing headers are propagated when ReportRateLimit call is performed