[prometheusremotewriteexporter] Use StartTimeUnixNano to export _created

[gouthamve]

We currently do not create {name}_created metric for counters and histograms. We should after open-telemetry/opentelemetry-specification#2645

[github-actions]

Pinging code owners: @Aneurysm9

[github-actions]

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Pinging code owners:

• exporter/prometheusremotewrite: @Aneurysm9

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[kovrus]

@Aneurysm9 please assign it to me