Improve snippets.py example for adding label data

[bws0013]

Is your feature request related to a problem? Please describe.

There aren't any examples of writing a timeseries which include writing label data in the python snippets. The metrics descriptor is created with a label [1] and a timeseries is written [2] but the time series doesn't include the label. This change would make it easier to understand how to push this (in python) data when following the gcp documentation [3] (which includes using these labels in go and C#).

[1] https://github.com/googleapis/python-monitoring/blob/2def0f3d66846d16fb58dd23fcf4a593ba084f7c/samples/snippets/v3/cloud-client/snippets.py#L39:L42

[2]

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

Line 60 in 2def0f3

def write_time_series(project_id):

[3] https://cloud.google.com/monitoring/custom-metrics/creating-metrics#writing-ts

Describe the solution you'd like

It would be useful to modify write_time_series to include writing the label as well.