Handle OTLP StartTime (similar to OpenMetrics Created timestamp)

[bboreham]

Proposal

The OTLP field StartTime / StartTimeUnixNano is described here. If I understand correctly, under Cumulative Temporality it could be used exactly like the Created timestamp in #6541, to add a 0 at the beginning of a series and hence avoid losing the first increment in rate().

(The only place I could find OTLP StartTime mentioned was at https://github.com/prometheus/prometheus/issues/13192, although it is only there incidentally)

[bwplotka]

Related nuanse https://github.com/open-telemetry/opentelemetry-specification/issues/4184 with the current CT zero injection and rates.

[beorn7]

Hello from the bug scrub!

This is being worked on.