feat: Add sample for asymmetric autoscaling instances

[hibiken]

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[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• samples/snippets/src/main/java/com/example/spanner/CreateInstanceWithAsymmetricAutoscalingConfigExample.java:19, tag spanner_create_instance_with_asymmetric_autoscaling_config

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