Provide a Manager metric value for when the system is balanced

[ddanielr]

Is your feature request related to a problem? Please describe.
Accumulo currently provides logs statements when the system is balancing or cannot balance due to outstanding migrations.
However, there is not an easy way to determine if the system is currently balanced and ready for work.

Describe the solution you'd like
The monitor should output a single metric value that matches the balanced vs not balanced system state.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
This metric would be used to determine if accumulo is "ready" after an accumulo cluster has been started.

[ddanielr]

This metric value could also be a count of tablets undergoing migration, and could report 0 when fully balanced.