feat: Adding labels to the metric descriptor in the snippets.py (#88)

This helps understand how to add labels for custom metrics [1].

[1] https://cloud.google.com/monitoring/custom-metrics/creating-metrics#create-metric-desc

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Fixes #86  🦕

samples/snippets/v3/cloud-client/snippets.py

@@ -19,6 +19,7 @@
 import uuid
 
 from google.api import metric_pb2 as ga_metric
+from google.api import label_pb2 as ga_label
 from google.cloud import monitoring_v3
 
 
@@ -34,6 +35,13 @@ def create_metric_descriptor(project_id):
     descriptor.metric_kind = ga_metric.MetricDescriptor.MetricKind.GAUGE
     descriptor.value_type = ga_metric.MetricDescriptor.ValueType.DOUBLE
     descriptor.description = "This is a simple example of a custom metric."
+
+    labels = ga_label.LabelDescriptor()
+    labels.key = "TestLabel"
+    labels.value_type = ga_label.LabelDescriptor.ValueType.STRING
+    labels.description = "This is a test label"
+    descriptor.labels.append(labels)
+
     descriptor = client.create_metric_descriptor(
         name=project_name, metric_descriptor=descriptor
     )