[TEST] Assert overflow attribute value is boolean true

[cijothomas]

Follow-up to #4060.

The spec says the overflow data point has exactly one attribute:
otel.metric.overflow -> bool true. Existing tests only checked the
key, so a regression in the value or its type would not be caught.
This PR adds the missing bool true assertions and pins the
precomputed kOverflowAttributes set against the spec.

Tests-only change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.09%. Comparing base (1711410) to head (2696323).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4063   +/-   ##
=======================================
  Coverage   82.09%   82.09%           
=======================================
  Files         385      385           
  Lines       15989    15989           
=======================================
  Hits        13124    13124           
  Misses       2865     2865           

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[lalitb]

Thanks @cijothomas - looking forward to more contributions from you ;)