StatsAccessLogger: fixes connection gauge underflow crashes when decrementing metrics after Scope evictions.

[TAOXUY]

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: Essentially we need to keep the gauge in the scope so that its value can be referenced to dec/inc. This PR introduce metric-level eviction disablement which allows us to safely decrement the gauge.

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

[ggreenway]

I don't think your fix is quite right.

I ran the integration test you added without your code changes, and it fails in an assertion ASSERT(used() || amount == 0); in sub(). I think either the assertion is no longer valid in the case of evicted stats, or the stat is being set to unused incorrectly.

      if (scope->evictable_) {
        MetricBag metrics(scope->scope_id_);
        CentralCacheEntrySharedPtr& central_cache = scope->centralCacheMutableNoThreadAnalysis();
        auto filter_unused = []<typename T>(StatNameHashMap<T>& unused_metrics) {
          return [&unused_metrics](std::pair<StatName, T> kv) {
            const auto& [name, metric] = kv;
            if (metric->used()) {
              metric->markUnused();
              return false;
            } else {
              unused_metrics.try_emplace(name, metric);
              return true;
            }
          };
        };

The above code assumes that a stat is only ever held by a single scope (or other holder of a reference), which isn't correct. cc @kyessenov .

I think the use of std::min around all the sub() calls means that it's likely the counter could be incorrect. Even if this change prevents it from going negative, I think it is still an incorrect count.

/wait

[TAOXUY]

I don't think your fix is quite right.

I ran the integration test you added without your code changes, and it fails in an assertion ASSERT(used() || amount == 0); in sub(). I think either the assertion is no longer valid in the case of evicted stats, or the stat is being set to unused incorrectly.

      if (scope->evictable_) {
        MetricBag metrics(scope->scope_id_);
        CentralCacheEntrySharedPtr& central_cache = scope->centralCacheMutableNoThreadAnalysis();
        auto filter_unused = []<typename T>(StatNameHashMap<T>& unused_metrics) {
          return [&unused_metrics](std::pair<StatName, T> kv) {
            const auto& [name, metric] = kv;
            if (metric->used()) {
              metric->markUnused();
              return false;
            } else {
              unused_metrics.try_emplace(name, metric);
              return true;
            }
          };
        };

The above code assumes that a stat is only ever held by a single scope (or other holder of a reference), which isn't correct. cc @kyessenov .

I think the use of std::min around all the sub() calls means that it's likely the counter could be incorrect. Even if this change prevents it from going negative, I think it is still an incorrect count.

/wait

Updated with a interface to not evict per metric. We need to keep gauge not evicted in the scope as that it can be looked-up and then dec/inc on the same gauge. @kyessenov

[TAOXUY]

/retest