Add monitoring to object cleaner

[opudrovs]

Part of weaveworks/weave-gitops-enterprise#3043

• Updated Explorer monitoring documentation to include the object cleaner metrics.

Notes:

• Not sure if we need such a large screenshot which shows all parts of the Explorer dashboard. If not, I can restore the old screenshot.
• Naming suggestions are welcome.

Questions:

• Do we agree on calling the cleaner component officially everywhere Objects Cleaner or Object Cleaner?

I saw it called differently in different places. For now, I am calling it Objects Cleaner.

[enekofb]

added minor suggestions

[opudrovs]

@enekofb neat suggestions, thanks!