Add tests for snapshotted trace-stats additional tags

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

Author: dougqh

internal-api/src/test/java/datadog/trace/api/TraceStatsAdditionalTagsTest.java

@@ -0,0 +1,52 @@
+package datadog.trace.api;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.Properties;
+import java.util.Set;
+import org.junit.jupiter.api.Test;
+
+class TraceStatsAdditionalTagsTest {
+
+  private static Config configWith(String experimentalFeatures, String additionalTags) {
+    Properties props = new Properties();
+    if (experimentalFeatures != null) {
+      props.setProperty("trace.experimental.features.enabled", experimentalFeatures);
+    }
+    if (additionalTags != null) {
+      props.setProperty("trace.stats.additional.tags", additionalTags);
+    }
+    return Config.get(props);
+  }
+
+  @Test
+  void returnsConfiguredTagsWhenFeatureEnabled() {
+    Config config = configWith("DD_TRACE_STATS_ADDITIONAL_TAGS", "peer.hostname,messaging.system");
+    Set<String> tags = config.getTraceStatsAdditionalTags();
+    assertEquals(2, tags.size());
+    assertTrue(tags.contains("peer.hostname"));
+    assertTrue(tags.contains("messaging.system"));
+  }
+
+  @Test
+  void emptyWhenFeatureNotEnabled() {
+    // The value is configured but the experimental feature gate is closed.
+    Config config = configWith(null, "peer.hostname");
+    assertTrue(config.getTraceStatsAdditionalTags().isEmpty());
+  }
+
+  @Test
+  void returnsSameSnapshotInstanceOnEachCall() {
+    // The setting is snapshotted into a final field at construction, so repeated reads
+    // return the identical immutable instance rather than re-querying the config source.
+    Config config = configWith("DD_TRACE_STATS_ADDITIONAL_TAGS", "peer.hostname");
+    assertEquals(config.getTraceStatsAdditionalTags(), config.getTraceStatsAdditionalTags());
+  }
+
+  @Test
+  void includedInToString() {
+    Config config = configWith("DD_TRACE_STATS_ADDITIONAL_TAGS", "peer.hostname");
+    assertTrue(config.toString().contains("traceStatsAdditionalTags="));
+  }
+}