Specialise validation for metric and event_name

[cpiemontese]

Trying to address #124

I was unsure how to address this and I opted for "specialising" the error message/validation base on what's being validated

This will "break" error messages in a sense, but it hopefully makes clearer what's not working (metric or event_name) and why

Happy to change the approach if it doesn't make sense

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.33%. Comparing base (ab86164) to head (4d583eb).
⚠️ Report is 11 commits behind head on main.
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Additional details and impacted files

@@             Coverage Diff             @@
##              main     #125      +/-   ##
===========================================
- Coverage   100.00%   99.33%   -0.67%     
===========================================
  Files            2        2              
  Lines          145      151       +6     
===========================================
+ Hits           145      150       +5     
- Misses           0        1       +1     

Files with missing lines	Coverage Δ
lib/telemetry_metrics.ex	98.97% <100.00%> (-1.03%)	⬇️

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[josevalim]

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