[enhancement] Add DynamicHostNameLabel feature

[bkranendonk]

This feature allows a operator to send their alerts to the ServiceHosts, based on the value of a Alert label.

Checklist

• [x] Keep pull requests small so they can be easily reviewed.
• [x] Update the README.
• [x] Categorize the PR by setting a good title and adding one of the labels: bug, enhancement, documentation, change, breaking, dependency as they show up in the changelog
• [x] Link this PR to related issues.

[friesoft]

Is this project still active? Last changes in june :(