chore(cogs): remove redundant "objectstore" from cogs metric name

[matt-codecov]

i forgot we prepend objectstore in the metric emitter config and don't need it in the metric name. whoops

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.36%. Comparing base (5c408cd) to head (8ad23c6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #498      +/-   ##
==========================================
- Coverage   87.37%   87.36%   -0.01%     
==========================================
  Files          86       86              
  Lines       13761    13761              
==========================================
- Hits        12023    12022       -1     
- Misses       1738     1739       +1     

Components	Coverage Δ
Rust Backend	92.20% <100.00%> (ø)
Rust Client	79.70% <ø> (ø)
Python Client	88.51% <ø> (ø)

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