[prometheus] Stabilize Prometheus -> OTLP: Start Time

[dashpole]

What are you trying to achieve?

Stabilize https://opentelemetry.io/docs/specs/otel/compatibility/prometheus_and_openmetrics/#start-time

We should probably delete the section, and add a note about start time to each of the typed sections where it is relevant. Instead of calling out _created series in particular, IMO we can be more generic, and say something like "The prometheus start (or created) timestamp MUST be added as the OpenTelemetry start time if it is present." For the OpenMetrics 1.0 text format, this is be represented by a separate series with the _created suffix.