Include regex syntax note in dogstatsd_mapper

[johannesschillinger-dm]

Clarify which regex dialect to use for metric name matching

What does this PR do? What is the motivation?

It was unclear, which regex dialect is supported by dogstatsd. This PR adds clarification

Merge instructions

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• Ready for merge

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Additional notes

[OliviaShoup]

hey, thank you for the PR! this branch is a fork, which means our usual checks in this repo can't finish running. without the checks, merging is blocked. because this is your first PR with us and it's only 1 line, i took your change and put it on my own branch here.

thanks again. we appreciate your contribution :)