sample(spanner): add set max commit delay sample

[NivedhaSenthil]

Description

Fixes #

Note: It's a good idea to open an issue first for discussion.

Checklist

• Tests pass
• Lint pass: bundle exec rubocop
• Please merge this PR for me once it is approved.

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• spanner/spanner_set_max_commit_delay.rb:15, tag spanner_set_max_commit_delay

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