[prometheusreceiver] Use _created to set StartTimeUnixNano

[gouthamve]

Currently we're using MetricsAdjuster to set the StartTimeUnixNano for the metrics. But when {name}_created is available we should use that.

See: #12426

[github-actions]

Pinging code owners: @dmitryax

[TylerHelmuth]

pinging @Aneurysm9 @dashpole as code owners

[dashpole]

We will also need to make some changes to the prom specification to support this:

• Prometheus exporters (including language + collector) SHOULD produce _created series for cumulative types, but this can be turned off.
• The prometheus receiver MUST drop the _created series, if present, and use it to set the start timestamp.

[dashpole]

I just noticed open-telemetry/opentelemetry-specification#2645, so no spec change is needed

[github-actions]

This issue has been inactive for 60 days. It will be closed in 60 days if there is no activity. To ping code owners by adding a component label, see Adding Labels via Comments, or if you are unsure of which component this issue relates to, please ping @open-telemetry/collector-contrib-triagers. If this issue is still relevant, please ping the code owners or leave a comment explaining why it is still relevant. Otherwise, please close it.

Pinging code owners:

• receiver/prometheus: @Aneurysm9 @dashpole

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[github-actions]

This issue has been inactive for 60 days. It will be closed in 60 days if there is no activity. To ping code owners by adding a component label, see Adding Labels via Comments, or if you are unsure of which component this issue relates to, please ping @open-telemetry/collector-contrib-triagers. If this issue is still relevant, please ping the code owners or leave a comment explaining why it is still relevant. Otherwise, please close it.

Pinging code owners:

• receiver/prometheus: @Aneurysm9 @dashpole

See Adding Labels via Comments if you do not have permissions to add labels yourself.