feat(spanner): add samples for instance partitions

[varuncnaik]

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Fixes #<issue_number_goes_here> 🦕

[snippet-bot]

Here is the summary of changes.

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• samples/samples/snippets.py:161, tag spanner_create_instance_partition

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[harshachinta]

Can we follow one of the 2 approaches,

1. We can re-use existing instance that is already created in

   python-spanner/samples/samples/snippets_test.py

   Line 149 in ec96d36

   deftest_create_instance_explicit(spanner_client, create_instance_id):

   .

This will prevent instance creation time when running tests and also reduce the no of instances that get created.
This test can be marked as dependent to the instance creation test like

python-spanner/samples/samples/snippets_test.py

Line 226 in ec96d36

@pytest.mark.dependency(name="create_instance_config")

.

2. If above one is complicated, then can we directly use snippets.create_instance to create an instance instead of rewriting the create logic here?

[varuncnaik]

I implemented Approach 2. The challenge with reusing an existing instance is that there are several restrictions around instance partitions. Notably:

• An instance partition cannot have the same instance config as the parent instance
• Several Spanner features do not work in instances where the user has created one or more instance partitions.