feat(firestore): add limit_to_last samples

[IlyaFaer]

Closes #4195

[IlyaFaer]

@BenWhitehead, @crwilcox, I've added a sample. Taken the tag name from the Java's samples

[gcf-merge-on-green]

Your PR has attempted to merge for 3 hours. Please check that all required checks have passed, you have an automerge label, and that all your reviewers have approved the PR

[BenWhitehead]

@IlyaFaer Looks like all three builds are failing with the following traceback

Traceback (most recent call last):
  File "/workspace/firestore/cloud-client/snippets_test.py", line 239, in test_order_limit_to_last
    snippets.order_limit_to_last()
  File "/workspace/firestore/cloud-client/snippets.py", line 579, in order_limit_to_last
    query = cities_ref.order_by("name").limit_to_last(2)
AttributeError: 'Query' object has no attribute 'limit_to_last'

[gcf-merge-on-green]

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[IlyaFaer]

@BenWhitehead, yeah, I suppose, it'll be failing until the next Firestore release. Locally I've replaced the Python original Firestore package with the last repository version, and it's working fine:

[tmatsuo]

let me know when it's ready to merge

[tmatsuo]

Does it make sense to check the output?

[busunkim96]

@IlyaFaer Could you comment on this?

You can return the results in the function and check them here if that's easier than checking the string output.

results = snippets.order_limit_to_last()
assert ...

[IlyaFaer]

Oh, that slipped my radars. Yeah, I thought about that, but most of the snippets tests are written this way - just a call to snippet function. So, as I see it, if we're gonna add an assert, we should do this for all the tests to keep the fashion, which probably should be done in a separate issue. Adding an assert into this single test, while other similar tests doesn't have it, looks weird. WDYT?

[tmatsuo]

Alright.

[tmatsuo]

Can you create an issue for it?

[IlyaFaer]

Sure, will make a /cc to you when created. And I think I'll do this

[busunkim96]

If you want to check that CI passes, you can temporarily change the requirements.txt to install GitHub from the master branch.

https://github.com/q-logic/python-docs-samples/blob/limit_to_last_samples/firestore/cloud-client/requirements.txt#L1

git+https://github.com/googleapis/python-firestore.git

[IlyaFaer]

@busunkim96, sounds interesting, thanks!
@tmatsuo, could you run kokoro checks? I don't have rights to add labels 😞
I've tried to change requirements.txt as adviced, let's give it a try

[BenWhitehead]

Kokoro builds triggered

[IlyaFaer]

@BenWhitehead, thanks! Checks are green. Please, acknowledge, and I'll revert requirements.txt
I've pinged the question of adding me into GoogleCloudPlatform, so, hopefully, I'll get some abilities soon

[IlyaFaer]

New Firestore client released, I've reverted requirements.txt changes, and now it's working fine.
Dropping "do not merge" label

[busunkim96]

@IlyaFaer Could you comment on this?

You can return the results in the function and check them here if that's easier than checking the string output.

results = snippets.order_limit_to_last()
assert ...

[IlyaFaer]

Friendly ping