Description: Fixes connection gauge underflow crashes in the Stats Access Logger when decrementing metrics after Scope evictions.

The original code correctly attempted to prevent "zombie" gauges by re-resolving metrics against the central store (via scope_->gaugeFromStatNameWithTags) during request destruction. However, it tried to reconstruct the gauge's identity using gauge_->tagExtractedStatName(). This failed because dynamic access-log tags (like %REQUEST_HEADER(...)%) are not registered with Envoy's global extractors. The extraction process returned a mangled base name and empty tags, forcing Scope to create a new 0-valued gauge. Subtracting 1 from it immediately crashed Envoy with a counter underflow.

Fix: we will keep the gauge in the scope cache if it is non-zero

Risk Level: Low

Testing: Added StatsAccessLogIntegrationTest.ActiveRequestsGaugeScopeEviction, which synthetically forces an asynchronous scope eviction while a connection is still inflight. Verified that the gauge successfully decrements to 0 in the central store identically to a normal request finish.

Docs: NA

Release: NA

Platform Specific Features: no

Benchmark result on single AddSubtract

Request variables

Key	Value
ref	02e5708
sha	9592318
pr	43812
base-sha	d3e80e6
actor	@TAOXUY
message	StatsAccessLogger: fixes connection gauge underflow crashes when decrementing metrics after Scope evictions....
started	1774479565.179137
target-branch	main
trusted	false

Container image/s (as used in this CI run)

Envoy version (as used in this CI run)