Add creation time of pull request to metric

[Thomsen96]

Hi,

My team would like to display the pull requests creation time on our dashboard.

Since it's already available in the response do I think its a nice and quick addition.

[sonarcloud]

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[mblaschke]

the creation date is already available as metric value.
eg in Grafana you can use this value to show the date or age (but might have to multiply it by 1000 because Grafana expects milliseconds and the exporter is exporting seconds)