nri-prom histogram improvement: add _count counter by default

[jackwilsdon]

Description

The _count counter is not included in data pushed to New Relic for Prometheus histograms.

Expected Behavior

The _count counter should be included.

Steps to Reproduce

Scrape a Prometheus histogram with the infrastructure agent and nri-prometheus. Note that the _count counter is not exported to New Relic.

Additional context

The summary type already has support for _count: https://github.com/newrelic/infrastructure-agent/blob/dc896c41ebea1cf7b15effb3cd05048e09a6bf70/pkg/integrations/v4/dm/telemetry.go#L270-L277

But it looks like the histogram type doesn't: https://github.com/newrelic/infrastructure-agent/blob/dc896c41ebea1cf7b15effb3cd05048e09a6bf70/pkg/integrations/v4/dm/telemetry.go#L201-L234

For Maintainers Only or Hero Triaging this bug

Suggested Priority (P1,P2,P3,P4,P5): Suggested T-Shirt size (S, M, L, XL, Unknown):

[workato-integration[bot]]

https://new-relic.atlassian.net/browse/NR-288815

[josemore]

this has been added to backlog as low-priority, see https://github.com/newrelic/infrastructure-agent/issues/1889#issuecomment-2376895704