Drain StatsMetrics singleton deltas after each test

StatsMetrics is a process-wide singleton; these tests left per-reason deltas
behind, leaking stats.collapsed_spans metrics into CoreMetricCollectorTest when
they shared a Gradle worker and making its exact span-metric count assertion
order-dependent. Reset every counter in @AfterEach.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dougqh

internal-api/src/test/java/datadog/trace/api/metrics/StatsMetricsTest.java

@@ -7,6 +7,7 @@
 import java.util.Map;
 import java.util.function.Function;
 import java.util.stream.Collectors;
+import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.Test;
 
 /**
@@ -20,6 +21,16 @@ private static Map<String, StatsMetrics.TaggedCounter> countersByTag() {
         .collect(Collectors.toMap(StatsMetrics.TaggedCounter::getTag, Function.identity()));
   }
 
+  @AfterEach
+  void drainDeltas() {
+    // StatsMetrics is a process-wide singleton; these tests leave per-reason deltas behind. Drain
+    // every counter so they do not leak into another collector's expectations when tests share a
+    // Gradle worker (e.g. CoreMetricCollectorTest asserting an exact span-metric count).
+    for (StatsMetrics.TaggedCounter counter : StatsMetrics.getInstance().getTaggedCounters()) {
+      counter.getValueAndReset();
+    }
+  }
+
   @Test
   void accumulatesPerReasonAndEmitsResetDeltas() {
     StatsMetrics metrics = StatsMetrics.getInstance();