Monitoring - Add plugin details and update dashboard.json

Checklist

[x] I read and understood the Contributing Guidelines.
[x] This is not a duplicate of an existing merge request.
[x] I believe this falls into the scope of the project and should be part of the built-in functionality.
[x] My code follows the code style of this project.
[x] I have added tests to cover my changes, wherever they are necessary.
[x] All new and existing tests pass.

Metric asf_active_plugins now has two additional time-series with the label type, which can have the values official and custom.
Included dashboard JSON file shows the newly added plugin metrics.
Included dashboard JSON file accounts for label changes in upstream libraries by calculating averages. The reason is that prometheus starts a new time-series every time a label changes. This had the effect that the colour of a graph would change at the point in time where the newest version was loaded the first time into ASF.

Thank you in advance for considering the inclusion of this merge request 😸