Ease extension of DataSourcePublicMetrics

[mduesterhoeft]

I want to publish additional DataSource metrics that a Tomcat DataSource provides (number of idle connections, current pool size).

Currently, this is very hard to do because the functionality in DataSourcePublicMetrics cannot be reused by subclasses.

I will come up with a proposal on how to improve this situation.

[wilkinsona]

Thanks for raising this. If you intend to immediately make a proposal in a pull request, there's no need to open an issue too. I'll close this one as a duplicate of the PR.

[wilkinsona]

Duplicate of #10289