enhancement(sample transform): Two tiny optimizations

[bruceg]

Summary

1. Avoid cloning the string generated by the group_by template when we can pass in ownership.
2. Defer generating the string value of the key_field until it is needed, which can be expensive if the underlying value is not a UTF-8 string.

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Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

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• No

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References

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