samples: Add snippets and samples for Count query

[Mariatta]

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Fixes #<issue_number_goes_here> 🦕

[snippet-bot]

Here is the summary of changes.

You are about to add 5 region tags.

• samples/snippets/snippets.py:82, tag datastore_count_in_transaction
• samples/snippets/snippets.py:114, tag datastore_count_on_kind
• samples/snippets/snippets.py:131, tag datastore_count_with_limit
• samples/snippets/snippets.py:149, tag datastore_count_with_property_filter
• samples/snippets/snippets.py:190, tag datastore_count_query_with_stale_read

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[Mariatta]

The mypy error is awaiting the next gapic-generator release.
It will get fixed with this change: googleapis/gapic-generator-python#1474

[dandhlee]

LGTM for samples, one minor nit.