code: func NewMetricWithTimestamp(t time.Time, m Metric) Metric {
  	return timestampedMetric{Metric: m, t: t}
  }
docs: // NewMetricWithTimestamp returns a new Metric wrapping the provided Metric in a
  // way that it has an explicit timestamp set to the provided Time. This is only
  // useful in rare cases as the timestamp of a Prometheus metric should usually
  // be set by the Prometheus server during scraping. Exceptions include mirroring
  // metrics with given timestamps from other metric
  // sources.
  //
  // NewMetricWithTimestamp works best with MustNewConstMetric,
  // MustNewConstHistogram, and MustNewConstSummary, see example.
  //
  // Currently, the exposition formats used by Prometheus are limited to
  // millisecond resolution. Thus, the provided time will be rounded down to the
  // next full millisecond value.