Improve LabelSource parsing

[prymitive]

No description provided.

[github-actions]

This pull request was validated by pint.

✔️ No problems found

Stats

Stat	Value
Version	v0.67.1-7-g1887ad0
Number of rules parsed	4
Number of rules checked	4
Number of problems found	0
Number of offline checks	0
Number of online checks	0
Checks duration	0

Problems

No problems reported

[github-actions]

Benchmark diff:

goos: linux
goarch: amd64
pkg: github.com/cloudflare/pint/cmd/pint
cpu: AMD EPYC 7763 64-Core Processor                
              │   old.txt   │              new.txt               │
              │   sec/op    │   sec/op     vs base               │
FindEntries-4   41.64m ± 3%   41.37m ± 3%       ~ (p=0.165 n=10)
CheckRules-4     8.531 ± 2%    8.544 ± 2%       ~ (p=0.971 n=10)
geomean         596.0m        594.5m       -0.25%

              │   old.txt    │               new.txt               │
              │     B/op     │     B/op      vs base               │
FindEntries-4   27.41Mi ± 0%   27.41Mi ± 0%       ~ (p=0.796 n=10)
CheckRules-4    967.0Mi ± 0%   966.8Mi ± 0%       ~ (p=0.684 n=10)
geomean         162.8Mi        162.8Mi       -0.01%

              │   old.txt   │              new.txt               │
              │  allocs/op  │  allocs/op   vs base               │
FindEntries-4   131.9k ± 0%   131.9k ± 0%       ~ (p=0.791 n=10)
CheckRules-4    14.10M ± 0%   14.10M ± 0%       ~ (p=0.436 n=10)
geomean         1.364M        1.364M       +0.00%

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.96%. Comparing base (3ad2373) to head (1887ad0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1181      +/-   ##
==========================================
+ Coverage   94.91%   94.96%   +0.05%     
==========================================
  Files         104      104              
  Lines       12360    12499     +139     
==========================================
+ Hits        11731    11870     +139     
  Misses        480      480              
  Partials      149      149              

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