code: func AddMetricTagsToPropagateCtx(
  	ctx context.Context,
  	tags map[string]string,
  ) context.Context {
  	return pcontext.AddToPropagateCtx(ctx, constants.MetricTagsKey, tags)
  }
docs: // AddMetricTagsToPropagateCtx adds a key and metric tags that will
  // be propagated through RPC calls. Use the same tags that are at
  // 'pitaya.metrics.additionalTags' config