Add metrics to improve observability of executor groups

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).