fix(metrics): Preserve backend tags on failed operations

[jan-auer]

When a backend call fails or times out, latency metrics lack backend tags because the tags were added only after success.

Backend tags are now set before storage operations run and updated when reads or deletes follow a long-term tombstone. Multipart latency metrics also include backend tags. PUT tags remain absent until its route is known.

Ref INC-2579

[linear-code]

INC-2579

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.34%. Comparing base (81dbdc8) to head (569b6bd).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #685      +/-   ##
==========================================
+ Coverage   91.33%   91.34%   +0.01%     
==========================================
  Files         117      117              
  Lines       24171    24179       +8     
==========================================
+ Hits        22077    22087      +10     
+ Misses       2094     2092       -2     

Components	Coverage Δ
Rust Backend	94.82% <100.00%> (+<0.01%)	⬆️
Rust Client	81.70% <ø> (ø)
Python Client	93.81% <ø> (ø)

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