refactor(collection): migrate metrics from /admin/mbeans to /admin/metrics API

Replace the deprecated /admin/mbeans endpoint with the Solr Metrics API
(/admin/metrics) for cache and handler statistics. The Metrics API is
node-level, so fetchMetrics now searches for the matching core registry
by prefix. Cache metrics remain nested NamedList objects while handler
metrics use flat dotted keys (e.g. QUERY./select.requests), requiring
separate extraction logic.

Key changes:
- fetchMetrics uses /admin/metrics with group=core and prefix filtering
- Handler extraction refactored to read flat keys via extractFlatHandlerInfo
- Core registry matching uses trailing dot to prevent false prefix matches
- Remove unused AVG_TIME_PER_REQUEST_FIELD and AVG_REQUESTS_PER_SECOND_FIELD
- Add integration tests for cache and handler metrics against real Solr
- Remove dead test helper createCompleteMockCacheData

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: adityamparikh <aditya.m.parikh@gmail.com>

Author: adityamparikh