CloudMonitoring: Fix crash when inspecting data for MQL queries (#63747)

CloudMonitoring: Fix missing Field name

This happened for MQL queries that used an [aggregate function][1], e.g.
`sum`.

[1]: https://cloud.google.com/monitoring/mql/reference#aggr-function-group

pkg/tsdb/cloudmonitoring/test-data/10-series-response-mql-no-labels.json

@@ -0,0 +1,39 @@
+{
+  "timeSeriesDescriptor": {
+    "pointDescriptors": [
+      {
+        "key": "value_utilization_sum",
+        "valueType": "DOUBLE",
+        "metricKind": "GAUGE"
+      }
+    ]
+  },
+  "timeSeriesData": [
+    {
+      "pointData": [
+        {
+          "values": [
+            {
+              "int64Value": "0"
+            }
+          ],
+          "timeInterval": {
+            "startTime": "2020-05-18T09:47:00Z",
+            "endTime": "2020-05-18T09:48:00Z"
+          }
+        },
+        {
+          "values": [
+            {
+              "int64Value": "0"
+            }
+          ],
+          "timeInterval": {
+            "startTime": "2020-05-18T09:46:00Z",
+            "endTime": "2020-05-18T09:47:00Z"
+          }
+        }
+      ]
+    }
+  ]
+}