feat(gax): correlate M3 client duration metrics with T3 span exemplars

Author: jinseopkim0

sdk-platform-java/gax-java/gax/src/main/java/com/google/api/gax/tracing/CompositeTracer.java

@@ -57,7 +57,10 @@ public Scope inScope() {
 
     try {
       for (ApiTracer child : children) {
-        childScopes.add(child.inScope());
+        Scope scope = child.inScope();
+        if (scope != null) {
+          childScopes.add(scope);
+        }
       }
     } catch (RuntimeException e) {
       for (int i = childScopes.size() - 1; i >= 0; i--) {
@@ -91,22 +94,28 @@ public Scope inScope() {
 
   @Override
   public void operationSucceeded() {
-    for (int i = children.size() - 1; i >= 0; i--) {
-      children.get(i).operationSucceeded();
+    try (Scope scope = inScope()) {
+      for (int i = children.size() - 1; i >= 0; i--) {
+        children.get(i).operationSucceeded();
+      }
     }
   }
 
   @Override
   public void operationCancelled() {
-    for (int i = children.size() - 1; i >= 0; i--) {
-      children.get(i).operationCancelled();
+    try (Scope scope = inScope()) {
+      for (int i = children.size() - 1; i >= 0; i--) {
+        children.get(i).operationCancelled();
+      }
     }
   }
 
   @Override
   public void operationFailed(Throwable error) {
-    for (int i = children.size() - 1; i >= 0; i--) {
-      children.get(i).operationFailed(error);
+    try (Scope scope = inScope()) {
+      for (int i = children.size() - 1; i >= 0; i--) {
+        children.get(i).operationFailed(error);
+      }
     }
   }