[prometheus components] For histograms without sums, leave the sum unset

[dashpole]

trafficstars

Description:

Prometheus histograms don't have a sum if the histogram includes negative observations. Now that the protocol supports a notion of "unset" sums, use this in prometheus components to correctly handle setting the sum.

I tried to do this for the prometheus exporter as well, but NewConstHistogram requires a sum (i.e. doesn't support an unset sum). https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/main/exporter/prometheusexporter/collector.go#L200

Link to tracking Issue:

Part of https://github.com/open-telemetry/opentelemetry-collector-contrib/issues/7546

Testing:

Added unit tests.

[dashpole]

cc @open-telemetry/wg-prometheus

[github-actions[bot]]

This PR was marked stale due to lack of activity. It will be closed in 14 days.

[github-actions[bot]]

This PR was marked stale due to lack of activity. It will be closed in 14 days.

[bogdandrutu]

@dashpole needs a rebase, sorry for the issue.