prometheus: bring back alerting configuration

This was dropped unintentionally during !36 (diffs)

requested review from @ibarrien

requested review from @ntsvetko

The price to pay is to add metrics.alertmanager.* to docs/values.md.

added 1 commit

• 34053b96 - docs: update alertmanager section

Compare with previous version

fair (and done)

approved this merge request

merged