[v10.0.x] CloudMonitoring: Improve parsing of GCM labels (#69812)

CloudMonitoring: Improve parsing of GCM labels (#69800)

Update parsing of GCM labels

- Include parsing of metadata labels

(cherry picked from commit 9fc1de62d5)

Co-authored-by: Andreas Christou <andreas.christou@grafana.com>

pkg/tsdb/cloudmonitoring/test-data/7-series-response-mql.json

@@ -13,6 +13,9 @@
       {
         "key": "metric.response_code_class",
         "valueType": "INT64"
+      },
+      {
+        "key": "metadata.app"
       }
     ],
     "pointDescriptors": [
@@ -37,6 +40,9 @@
         },
         {
           "int64Value": "200"
+        },
+        {
+          "stringValue": "test-app"
         }
       ],
       "pointData": [