Describe the enhancement:
Implement a single metricset to collect Prometheus metrics from istiod. Implementation can be as a light-weight module on top of Prometheus module. Part of #15505.

See #15505 (comment)

Describe a specific use case for the enhancement or feature:
This metricset will be the way to monitor Istio after v1.5 where the various microservices have been converged into a single monolithic service called istiod.