juju prometheus metrics should include a build_info metric

[John A Meinel]

We want an easy way to track what version is being run alongside all the other metrics that are being rendered.
That way you can see very clearly the pre and post values. (what version was this controller running during this outage from 2 days ago, etc)

Long term it might be nice to have aggregated 'what versions are agents running' but just having the versions for the controllers would be a big win.

Note the recommendations from Prometheus
https://prometheus.io/docs/instrumenting/writing_exporters/#target-labels-not-static-scraped-labels

The library itself has a NewBuildInfoCollector
https://github.com/prometheus/client_golang/blob/2c3d072cdd4dbd9dbbfe415cbd01851db8962289/prometheus/build_info_collector.go#L22

However it pretty much only grabs the golang version information, vs the other information that Juju exposes.

[John A Meinel (jameinel)]

https://github.com/juju/juju/pull/14134