[prometheus] Stabilize Prometheus -> OTLP: Histograms

[dashpole]

What are you trying to achieve?

Stabilize https://opentelemetry.io/docs/specs/otel/compatibility/prometheus_and_openmetrics/#histograms

It would be helpful to re-phrase this section to not rely as heavily on text format suffixes. E.g.

Lines with _count and _sum suffixes are used to determine the histogram’s count and sum.

Should become something like:

The histogram count must be used to determine the histogram's count. For the Prometheus text format, the count is a separate series of the form <metric family name>_count.