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 #13192, although it is only there incidentally)

[bwplotka]

Related nuanse open-telemetry/opentelemetry-specification#4184 with the current CT zero injection and rates.