Add more metric trigger documentation

[discdiver]

First check

• [X] I added a descriptive title to this issue.
• [X] I used GitHub search to find a similar request and didn't find it 😇

Describe the issue

Metric trigger documentation is pretty light.

Describe the proposed change

Add more information about Metric trigger to Automations concept docs.

Additional context

Should reference the guide proposed in #11626 when completed.