Add Busy/Idle metric for compactors

[ddanielr]

Is your feature request related to a problem? Please describe.
When performing a scale-in operation, only compactors that are not performing compactions should be candidates for removal.
Currently, a combination of using jstack and looking for a MajC thread can help determine if a compactor is performing work.

Describe the solution you'd like
The compactor should emit a metric value to determine if it's currently working or idle.
This metric can then be used to drive scaling decisions and/or rolling compactor updates.

[DomGarguilo]

I can take a look at this