Merges pratik/otel-phase8-log-correlation into pratik/otel-phase9-metric-gap-fill.
Auto-merged. TESTING.md picked up phase-8's Test 4 rewrite alongside this
branch's Step 2 standalone-store fix. OTelCollector.cpp gained the weak_ptr
gauges_ list; the StatsD test gained its onCollectionReady() calls.