feat: Create a few code snippets as examples for using Spanner Graph in Python

[bharadwajvr]

Add a few code snippets to illustrate how to use Spanner Graph functionality in Python. These examples resemble other snippets for how various other Spanner features can be used.

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes#1185 🦕

[snippet-bot]

Here is the summary of changes.

You are about to add 9 region tags.

• samples/samples/graph_snippets.py:30, tag spanner_create_database_with_property_graph
• samples/samples/graph_snippets.py:102, tag spanner_insert_graph_data
• samples/samples/graph_snippets.py:162, tag spanner_insert_graph_data_with_dml
• samples/samples/graph_snippets.py:197, tag spanner_update_graph_data_with_dml
• samples/samples/graph_snippets.py:226, tag spanner_update_graph_data_with_graph_query_in_dml
• samples/samples/graph_snippets.py:251, tag spanner_query_graph_data
• samples/samples/graph_snippets.py:272, tag spanner_query_graph_data_with_parameter
• samples/samples/graph_snippets.py:297, tag spanner_delete_graph_data_with_dml
• samples/samples/graph_snippets.py:324, tag spanner_delete_graph_data

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[bharadwajvr]

I did not find instructions for this in the authoring guide - how do I obtain code coverage information for the new code I've written? There is an ask in the description to verify that " Code coverage does not decrease (if any source code was changed)"

[bharadwajvr]

Gentle ping.

[bharadwajvr]

A reminder

[harshachinta]

@bharadwajvr Thanks for the PR. Will review it early next week.

[harshachinta]

I did not find instructions for this in the authoring guide - how do I obtain code coverage information for the new code I've written? There is an ask in the description to verify that " Code coverage does not decrease (if any source code was changed)"

Looking at the files changed, there is no source file changes in this PR. So this can be ignored.

[bharadwajvr]

@bharadwajvr Thanks for the PR. Will review it early next week.

Thank you @harshachinta

[harshachinta]

Adding Do not merge to check the presubmit build status before merging

[harshachinta]

@bharadwajvr
Can you please fix the failing lint

[bharadwajvr]

@bharadwajvr Can you please fix the failing lint

Done. Local run succeeds. @harshachinta

$ flake8 --show-source --builtin=gettext --max-complexity=20 '--exclude=.nox,.cache,env,lib,generated_pb2,*_pb2.py,*_pb2_grpc.py' --ignore=E121,E123,E126,E203,E226,E24,E266,E501,E704,W503,W504,I202 --max-line-length=88
$