Snapshot trace-stats additional tags in Config

getTraceStatsAdditionalTags re-queried ConfigProvider on every call, so a late
system-property mutation could change an existing Config's value, and the
setting never appeared in Config.toString() (nor was it resolved at all when no
metrics aggregator was constructed). Resolve it once into a final field during
construction, return that field, and include it in toString(), per the
add_new_configurations convention.

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

Author: dougqh

internal-api/src/main/java/datadog/trace/api/Config.java

@@ -941,6 +941,7 @@ public static String getHostName() {
   private final boolean dbClientSplitByInstanceTypeSuffix;
   private final boolean dbClientSplitByHost;
   private final Set<String> splitByTags;
+  private final Set<String> traceStatsAdditionalTags;
   private final boolean jeeSplitByDeployment;
   private final int scopeDepthLimit;
   private final boolean scopeStrictMode;
@@ -1827,6 +1828,12 @@ private Config(final ConfigProvider configProvider, final InstrumenterConfig ins
 
     splitByTags = tryMakeImmutableSet(configProvider.getList(SPLIT_BY_TAGS));
 
+    traceStatsAdditionalTags =
+        experimentalFeaturesEnabled.contains(
+                propertyNameToEnvironmentVariableName(TRACE_STATS_ADDITIONAL_TAGS))
+            ? tryMakeImmutableSet(configProvider.getList(TRACE_STATS_ADDITIONAL_TAGS))
+            : Collections.emptySet();
+
     jeeSplitByDeployment =
         configProvider.getBoolean(
             EXPERIMENTATAL_JEE_SPLIT_BY_DEPLOYMENT, DEFAULT_EXPERIMENTATAL_JEE_SPLIT_BY_DEPLOYMENT);
@@ -5288,11 +5295,7 @@ public Set<String> getMetricsIgnoredResources() {
   }
 
   public Set<String> getTraceStatsAdditionalTags() {
-    if (!experimentalFeaturesEnabled.contains(
-        propertyNameToEnvironmentVariableName(TRACE_STATS_ADDITIONAL_TAGS))) {
-      return Collections.emptySet();
-    }
-    return tryMakeImmutableSet(configProvider.getList(TRACE_STATS_ADDITIONAL_TAGS));
+    return traceStatsAdditionalTags;
   }
 
   public String getEnv() {
@@ -6435,6 +6438,8 @@ public String toString() {
         + dbmTracePreparedStatements
         + ", splitByTags="
         + splitByTags
+        + ", traceStatsAdditionalTags="
+        + traceStatsAdditionalTags
         + ", jeeSplitByDeployment="
         + jeeSplitByDeployment
         + ", scopeDepthLimit="