docs: @Nullable
    private Status validateInitialMetadata(Metadata headers) {
      Integer httpStatus = headers.get(HTTP2_STATUS);
      if (httpStatus == null) {
        return Status.INTERNAL.withDescription("Missing HTTP status code");
      }
      String contentType = headers.get(GrpcUtil.CONTENT_TYPE_KEY);
      if (!GrpcUtil.isGrpcContentType(contentType)) {
        return GrpcUtil.httpStatusToGrpcStatus(httpStatus)
            .augmentDescription("invalid content-type: " + contentType);
      }
      return null;
    }