GRPC Tracing Dropping trailers

[scott-clary]

My GRPC Functions are defined as
function (call: grpc.ServerUnaryCall<any, any> | grpc.ServerReadableStream<any, any>, callback: grpc.sendUnaryData<any>) {

In them I return the callback with trailers added
return callback(null, {}, trailers);

Note sendUnaryData type

export type sendUnaryData<ResponseType> = (error: ServerErrorResponse | ServerStatusResponse | null, value?: ResponseType | null, trailer?: Metadata, flags?: number) => void;

Unfortunately in the experimental/packages/opentelemetry-instrumentation-grpc/src/serverUtils.ts its using the
SendUnaryDataCallback type which doesn't include the trailer or flags.

/**
 * Server Unary callback type
 */
export type SendUnaryDataCallback<T> = requestCallback<T>;

export interface UnaryCallback<ResponseType> {
    (err: ServiceError | null, value?: ResponseType): void;
}

/**
 * Handle patching for clientStream and unary type server handlers
 */
function clientStreamAndUnaryHandler<RequestType, ResponseType>(
  span: Span,
  call: ServerCallWithMeta<RequestType, ResponseType>,
  callback: SendUnaryDataCallback<ResponseType>,
  original:
    | handleUnaryCall<RequestType, ResponseType>
    | ClientReadableStream<RequestType>
): void {
  const patchedCallback: SendUnaryDataCallback<ResponseType> = (
    err: ServiceError | null,
    value?: ResponseType
  ) => {
    if (err) {
      if (err.code) {
        span.setStatus({
          code: _grpcStatusCodeToOpenTelemetryStatusCode(err.code),
          message: err.message,
        });
        span.setAttribute(SEMATTRS_RPC_GRPC_STATUS_CODE, err.code);
      }
      span.setAttributes({
        [AttributeNames.GRPC_ERROR_NAME]: err.name,
        [AttributeNames.GRPC_ERROR_MESSAGE]: err.message,
      });
    } else {
      span.setStatus({ code: SpanStatusCode.UNSET });
      span.setAttribute(SEMATTRS_RPC_GRPC_STATUS_CODE, GRPC_STATUS_CODE_OK);
    }

    span.end();
    return callback(err, value);
  };

All this to say is this function should probably return trailers and flags in the callback, otherwise they are lost when tracing is turned on.