Support metrics 0.15.1

[DifferentialOrange]

Support metrics introduced in metrics 0.15.1. See commit messages for related info.

Closes #197

If this pull request introduces support for new metrics, do not forget to

• [x] add Grafana panel;
• [x] run make update-tests to build new dashboard artifacts;
• [x] add example application code to generate non-null metrics;
• [x] attach a screenshot of a new panel to the PR description;
• [x] add new Telegraf tag key, if relevant,
  • [x] to all telegraf.conf files,
  • [x] to "Grafana dashboard" documentation page;
• add alert example, if relevant,
  • to alerts.yml,
  • to "Alerting" documentation page;
• [x] update supported_metrics.md.

In any case, do not forget to

• [x] link an issue;
• [x] add a CHANGELOG entry;
• update documentation guidelines;
• update README guidelines.