code: private static boolean isApplicationHeader(String key) {
      // Don't allow HTTP/2 pseudo headers or content-type to be added by the application.
      return (!key.startsWith(":")
              && !CONTENT_TYPE_KEY.name().equalsIgnoreCase(key))
              && !USER_AGENT_KEY.name().equalsIgnoreCase(key);
    }
docs: Returns {@code true} if the given header is an application-provided header. Otherwise, returns
  {@code false} if the header is reserved by GRPC.