Store timestamp of last pushed metric

[elecnix]

Scraping pushprom returns no timestamp for metrics, which may be very old. No staleness check is possible unless one metric contains a timestamp as a value.

Instead, pushprom could provide the timestamp when the metric was received.

The wire format allows it: https://prometheus.io/docs/instrumenting/exposition_formats/

[samwierema]

We can't currently do this at this time, the Go client for Prometheus explicitly doesn't allow it. See this issue for more information: https://github.com/prometheus/client_golang/issues/143

[marcelcorso]

If you want to check for the "health" of a metric an idea is to look at rate it's changing... Not sure.