prometheus-exporter: export measure

[vmarchaud]

The exporter do not export measure which has been reported over gitter. Note that each measure intrument is aggregated as follow:

{
  min: Infinity,
  max: -Infinity,
  sum: 0,
  count: 0,
}

While waiting for the View API which will allow measure to be aggregated following user configuration, i think we should compute an avg from sum / count and export that. WDYT ?

cc @paulfairless @open-telemetry/javascript-approvers

[paulfairless]

Thank you for opening this. My main objective using the Measure type was to record function timings, short of specific instrumentation methods to do so. I believe the Summary type in Prometheus may be suitable for this data.

A simple scenario would record the sum and count in the Summary.
A more advanced case could include min, max and average as percentiles, though I believe the use of Summary percentiles is no longer advised as it poses aggregation problems. I believe this would be a similar problem if we exposed an average.

[vmarchaud]

Closing both the PR and this issue since the measure as been replaced by ValueRecorderMetric