You cannot tag individual metrics. The correct way to create alerts for your custom metrics is to use the "Custom events for alerting" screen as detailed in this blog post: https://www.dynatrace.com/news/blog/leverage-the-power-of-davis-ai-with-custom-time-series-events-fo...
@Sascha K. If you are looking to automatically tag these custom plugins, you can do it by following these simple steps:
1.) Go to Settings > Tags > Automatically Applied Tags > Click "Create Tag"
2.) Once at the tag creation page, create a tag name. Keep in mind, this wont be the total value of the tagged host. Then supply the optional tag values. A great example of result would be if you want to tag hosts by States. you have hosts in NY and Florida. We would supply the tag name as "Location" And supply a Value of NY or Florida. So when it automatically tags a host, it will appear as "Location: NY"
Select the scope of what this rule applies to. In your case we will use Custom Devices. We can then narrow the scope of this tag by the technology, so we could narrow it down to just Java if we'd like.
Then Select the conditions. Basically if the the defined condition is/not present, then tag it with the tagged value as defined.
Don't forget to click preview to ensure that the tags are configured as desired. Once finished click "Create Rule"
Let me know if you need any assistance!