As a follow on
IMPALA-8484, it makes sense to add some metrics to provide better observability into the state of executor groups.
Some metrics can be:
- number of executor groups with any impalas in them
- number of healthy executor groups
- number of backends. Currently we have a python helper that calculates this - get_num_known_live_backends, but it really should be a metric (we could replace the test code with a metric if we did this).