Add balancer meter fixes #4470

[EdColeman]

Adds two gauges that can be used to monitor balancing

• manager.balancer.balancing.total - snapshot of the number of current migrations being performed.
• manager.balancer.need.balancing.total - snapshot of the total number of migrations needed to balancing.

This fixes #4470

[EdColeman]

I confirmed that with uno instance configured to use a LoggingProvider, the meters appear in the manager metrics log:

METRICS: 2024-04-19T12:08:37,303, manager.balancer.balancing.total{host=localhost,instance.name=uno,port=9999,process.name=manager} value=0
METRICS: 2024-04-19T12:08:37,303, manager.balancer.need.balancing.total{host=localhost,instance.name=uno,port=9999,process.name=manager} value=0

[ddanielr]

Closed in favor of #4699