fix(spanner): resolve TypeError in metrics resource detection

Author: sinhasubham

google/cloud/spanner_v1/metrics/spanner_metrics_tracer_factory.py

@@ -169,4 +169,4 @@ def _get_location() -> str:
         if GOOGLE_CLOUD_REGION_KEY not in resources.attributes:
             return GOOGLE_CLOUD_REGION_GLOBAL
         else:
-            return resources[GOOGLE_CLOUD_REGION_KEY]
+            return resources.attributes[GOOGLE_CLOUD_REGION_KEY]