Make Advice proper immutable class

[bogdandrutu]

There may be other places like this, @open-telemetry/java-maintainers you can fix them if you want.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01 🎉

Comparison is base (ac0b4e4) 91.34% compared to head (be325b2) 91.36%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5532      +/-   ##
============================================
+ Coverage     91.34%   91.36%   +0.01%     
- Complexity     4957     4959       +2     
============================================
  Files           550      550              
  Lines         14543    14546       +3     
  Branches       1359     1359              
============================================
+ Hits          13285    13290       +5     
+ Misses          872      871       -1     
+ Partials        386      385       -1     

Impacted Files	Coverage Δ
...emetry/sdk/metrics/internal/descriptor/Advice.java	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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