message: cleanup: Consistent arg names in Envoy::Grpc::Common.
  getGrpc{Status,Message} had a parameter named `headers` in common.h but
  `trailers` in common.cc. I confirmed with that `trailers` was
  correct.
diff: @@ -27,20 +27,20 @@ public:
  class Common {
  public:
  /**
  - * Returns the GrpcStatus code from a given set of headers, if present.
  - * @param headers the headers to parse.
  + * Returns the GrpcStatus code from a given set of trailers, if present.
  + * @param trailers the trailers to parse.
  * @return Optional<Status::GrpcStatus> the parsed status code or InvalidCode if no valid status
  * is found.
  */
  - static Optional<Status::GrpcStatus> getGrpcStatus(const Http::HeaderMap& headers);
  + static Optional<Status::GrpcStatus> getGrpcStatus(const Http::HeaderMap& trailers);
  /**
  - * Returns the grpc-message from a given set of headers, if present.
  - * @param headers the headers to parse.
  + * Returns the grpc-message from a given set of trailers, if present.
  + * @param trailers the trailers to parse.
  * @return std::string the gRPC status message or empty string if grpc-message is not present in
  - * headers.
  + * trailers.
  */
  - static std::string getGrpcMessage(const Http::HeaderMap& headers);
  + static std::string getGrpcMessage(const Http::HeaderMap& trailers);
  /**
  * Returns the gRPC status code from a given HTTP response status code. Ordinarily, it is expected
  