SpringBoot Expose Git and build information as metrics

Introduce new metrics git.info and build.info that expose some information from GitProperties and BuildProperties respectively, if available. This is the same info that /actuator/info provides, but represented as metrics, which allows for convenient integration in Grafana dashboards.

Using tags on a pseudo gauge seems to be the standard way of doing this; see for example Prometheus' go_info metric or Micrometer's own jvm.info metric (not yet included in Spring Boot).

Comment From: mafr

There were some formatting issue that I'm going to fix on a separate PR.

Comment From: snicoll

@mafr Going forward, please don't do that. Pushing additional commits on mafr:project-info-metrics will update this PR and make sure we don't end up with several PRs for the same thing.