[runtime] Normalize labels

[syrupipy]

Closes #1783 by normalizing metric labels to replace '-' with '_' for Prometheus compatibility.

[patrick-ogrady]

Can you add a test for this?

I'm still sorting out whether we want to normalize or panic.

[syrupipy]

Can you add a test for this?

Sure added now

[codecov]

Codecov Report

❌ Patch coverage is 96.55172% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.17%. Comparing base (33a3a75) to head (9a2a9db).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
runtime/src/lib.rs	95.65%	1 Missing ⚠️

@@            Coverage Diff             @@
##             main    #1785      +/-   ##
==========================================
+ Coverage   92.16%   92.17%   +0.01%     
==========================================
  Files         302      302              
  Lines       77817    77840      +23     
==========================================
+ Hits        71722    71751      +29     
+ Misses       6095     6089       -6     

Files with missing lines	Coverage Δ
runtime/src/deterministic.rs	96.80% <100.00%> (+<0.01%)	⬆️
runtime/src/tokio/runtime.rs	84.04% <100.00%> (+0.04%)	⬆️
runtime/src/lib.rs	97.54% <95.65%> (+0.56%)	⬆️

... and 1 file with indirect coverage changes

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