CheckRateLimit Tracing Headers

[eguzki]

Following the introduction of the new CheckRateLimit gRPC method in #430, we have an opportunity to provide richer tracing to rate-limited clients.

service RateLimitService {
  // Determine whether a request should be allowed or denied based on rate limiting rules.
  rpc CheckRateLimit(envoy.service.ratelimit.v3.RateLimitRequest) returns (envoy.service.ratelimit.v3.RateLimitResponse); 
}

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