Implement a /admin/metrics API

We added a bare bones metrics API in SOLR-9788 but due to limitations with the metrics servlet supplied by the metrics library, it can show statistics from only one metric registry. SOLR-4735 has added a hierarchy of metric registries and the /admin/metrics API should support showing all of them as well as be able to filter metrics from a given registry name.

In this issue we will implement the improved /admin/metrics API.