Fix dictionary changed size during iteration in prometheus

[bdraco]

Proposed change

The code inside the loop deletes items in the container

Found this while looking for RuntimeError in issues and I figured it was easy to fix so I did.

fixes #104803

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[home-assistant]

Hey there @knyar, mind taking a look at this pull request as it has been labeled with an integration (prometheus) you are listed as a code owner for? Thanks!

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[knyar]

Thank you!

[bdraco]

thanks