RFR: metrics: add Prometheus exporter

[Erik Joelsson]

On Wed, 12 May 2021 12:50:26 GMT, Erik Helin <ehelin at openjdk.org> wrote:

> Hi all,
> 
> please review this patch that adds a Prometheus exporter to the `metrics` package. The Prometheus format for metrics can be seen [here](https://prometheus.io/docs/instrumenting/exposition_formats/#text-format-details). I also added a bunch of unit tests for the exporter.
> 
> Thanks,
> Erik

Marked as reviewed by erikj (Reviewer).

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PR: https://git.openjdk.java.net/skara/pull/1150