fix: prevent permanent merging of labels

[arsanjea]

Fixes the permanent merging of labels after logging a message with custom labels.

Fixes #88

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

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[arsanjea]

I signed it!

[googlebot]

CLAs look good, thanks!

[ofrobots]

Hi @arsanjea. Thank you very much for your contribution, and sorry about the bug. Thanks for fixing it.

We can go ahead and land this once we have added some tests and fix the format errors in the CI. We will take care of it, but if you want to contribute those changes, that is welcome too.

[arsanjea]

Awesome. Great turnaround.
This package is helping me centralize all my micro services log in one place.
Happy to give back.