Add filter for stats name

[rthouvenin]

Pull Request Checklist

Implements #43. Adds a command line option to metrics-per-process.py to filter with a regexp the stats to output. Prints everything by default for backward compatibility. The stats that are filtered out are not queried, which avoids getting permission errors for stats that are not required.

General

• [x] Update Changelog following the conventions laid out on Keep A Changelog

• [ ] Update README with any necessary configuration snippets

• N/A Binstubs are created if needed

• N/A RuboCop passes

• N/A Existing tests pass

[rthouvenin]

Here is a gist with the output of a few test runs at this stage: https://gist.github.com/rthouvenin/f43ab687b85c7543b781668a3ac7b8a2

[majormoses]

released: https://rubygems.org/gems/sensu-plugins-process-checks/versions/2.5.0