feat: catch, discard & warn about the labels that have reserved names

[olegbespalov]

What?

Cloud Output V2 drops labels that are reserved, for now, we're sticking with the test_run_id and Prometheus system metrics (everything that prefixed with __)

Why?

These tags are reserved for internal use and can't be set with the client.

Checklist

• I have performed a self-review of my code.
• I have added tests for my changes.
• I have run linter locally (make ci-like-lint) and all checks pass.
• I have run tests locally (make tests) and all tests pass.
• I have commented on my code, particularly in hard-to-understand areas.

Related PR(s)/Issue(s)

Closes #3155

[codebien]

It requires minimizing the allocation because we invoke this code with high frequency.

[codecov-commenter]

Codecov Report

Merging #3162 (767fba9) into master (aa79387) will increase coverage by 0.05%.
The diff coverage is 90.90%.

❗ Current head 767fba9 differs from pull request most recent head 327ab5a. Consider uploading reports for the commit 327ab5a to get more accurate results

@@            Coverage Diff             @@
##           master    #3162      +/-   ##
==========================================
+ Coverage   72.78%   72.84%   +0.05%     
==========================================
  Files         255      255              
  Lines       19585    19611      +26     
==========================================
+ Hits        14255    14285      +30     
+ Misses       4433     4430       -3     
+ Partials      897      896       -1     

Flag	Coverage Δ
ubuntu	72.78% <90.90%> (+0.05%)	⬆️
windows	72.67% <90.90%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
output/cloud/expv2/flush.go	90.16% <89.47%> (+3.20%)	⬆️
output/cloud/expv2/mapping.go	94.80% <91.66%> (+0.68%)	⬆️
output/cloud/expv2/output.go	66.45% <100.00%> (+0.43%)	⬆️

... and 2 files with indirect coverage changes

[codebien]

LGTM 🙇 Just a minor on the warn message.

[codebien]

Just caught only now. We can just use a slice storing the tags' keys during the detection process. A TagSet shouldn't include multiple tags with the same key.

[olegbespalov]

@esquonk, adding you also as the reviewer, probably the most interesting part for you is https://github.com/grafana/k6/pull/3162/files#diff-2edc8a9a0a0d4ec2a88767baae9d8ad5c035168eeee41b7b5c8b36ce31d9ce70R42-R49

[oleiade]

👏🏻 🙇🏻