feat: Add async sample for incrementing a value

[KasiaStrz]

Fixes b/452032333.

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• samples/snippets/writes/write_increment_async.py:1, tag bigtable_write_increment_async

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[chalmerlowe]

LGTM

[daniel-sanche]

Actually, looking at it again, are you sure this is doing what you want? This new sample isn't just an async version of write_increment, AddToCell is related to aggregates, which is something totally different.

If this sample is just for incrementing a value, we can probably close this, because we have that sample here

If this is supposed to be an AddToCell/aggregation sample, we should give this a different name to be more distinct, and probably move this to the data_client folder with the others. And we may have to do some work in the test project, because I don't think the test cluster is currently set up to work with aggregate families

Sorry for not catching this earlier. Let me know how you want to proceed

[daniel-sanche]

Following up offline, this is intended to be an aggregation test. So we'll need to:

• change the region tag to bigtable_writes_aggregate
• move the tests to the data folder
• set up test families that are configured for aggregations

I'm going to take over this PR to get the last pieces through (Thanks @KasiaStrz!)

[daniel-sanche]

Closing in favor of #1285