Need to add timestamp in data series before reporting to prometheus

[winattack]

Is your feature request related to a problem? Please describe.
default timestamp mechanism will lead to wrong statistics due to network shaking

Describe the solution you'd like
like go client, add NewMetricWithTimestamp support

// Create a constant gauge from values we got from an external
// temperature reporting system. Those values are reported with a slight
// delay, so we want to add the timestamp of the actual measurement.
temperatureReportedByExternalSystem := 298.15
timeReportedByExternalSystem := time.Date(2009, time.November, 10, 23, 0, 0, 12345678, time.UTC)
s := prometheus.NewMetricWithTimestamp(
timeReportedByExternalSystem,
prometheus.MustNewConstMetric(
desc, prometheus.GaugeValue, temperatureReportedByExternalSystem,
),
)