feat: enable pre-emptive async oauth token refreshes

oauth2_http/java/com/google/auth/oauth2/ExternalAccountCredentials.java

@@ -35,6 +35,7 @@
 
 import com.google.api.client.json.GenericJson;
 import com.google.api.client.json.JsonObjectParser;
+import com.google.auth.RequestMetadataCallback;
 import com.google.auth.http.HttpTransportFactory;
 import com.google.auth.oauth2.AwsCredentials.AwsCredentialSource;
 import com.google.auth.oauth2.IdentityPoolCredentials.IdentityPoolCredentialSource;
@@ -48,6 +49,7 @@
 import java.util.Collection;
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.Executor;
 import javax.annotation.Nullable;
 
 /**
@@ -208,6 +210,27 @@ private ImpersonatedCredentials initializeImpersonatedCredentials() {
         .build();
   }
 
+  @Override
+  public void getRequestMetadata(
+      URI uri, Executor executor, final RequestMetadataCallback callback) {
+    super.getRequestMetadata(
+        uri,
+        executor,
+        new RequestMetadataCallback() {
+          @Override
+          public void onSuccess(Map<String, List<String>> metadata) {
+            metadata = addQuotaProjectIdToRequestMetadata(quotaProjectId, metadata);
+            callback.onSuccess(metadata);
+          }
+
+          @Override
+          public void onFailure(Throwable exception) {
+            callback.onFailure(exception);
+          }
+        });
+    super.getRequestMetadata(uri, executor, callback);
+  }
+
   @Override
   public Map<String, List<String>> getRequestMetadata(URI uri) throws IOException {
     Map<String, List<String>> requestMetadata = super.getRequestMetadata(uri);