docs(samples): include example writing of label data (#202)

adding label example when writing a timeseries in python

Remaking this due to the commit lint failure in #200 and it being easier to just make a new PR.  Closing #200. 

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Fixes #200  🦕

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

@@ -67,6 +67,7 @@ def write_time_series(project_id):
     series.resource.type = "gce_instance"
     series.resource.labels["instance_id"] = "1234567890123456789"
     series.resource.labels["zone"] = "us-central1-f"
+    series.metric.labels["TestLabel"] = "My Label Data"
     now = time.time()
     seconds = int(now)
     nanos = int((now - seconds) * 10 ** 9)