Add readme section on metric label collisions

[damemi]

Fixes #352
Document how to avoid metric label collisions with the transform processor, like https://cloud.google.com/stackdriver/docs/managed-prometheus/setup-otel#rename-resource-attributes

[dashpole]

LGTM. Lets make sure this gets copied upstream as well, as users will look at that more often

[codecov]

Codecov Report

Merging #681 (982d7b6) into main (f8434b1) will not change coverage.
The diff coverage is n/a.

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##             main     #681   +/-   ##
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  Coverage   69.54%   69.54%           
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  Files          36       36           
  Lines        4699     4699           
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  Hits         3268     3268           
  Misses       1278     1278           
  Partials      153      153           

see 1 file with indirect coverage changes

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[damemi]

LGTM. Lets make sure this gets copied upstream as well, as users will look at that more often

Sounds good. I also just added an update clarifying that the user's attributes might have . instead of _, but will still conflict once normalized, and to update the config if so