docs: fix unit in metrics semantic conventions example

[neoeinstein]

Changes

Minor documentation update for a code example. Duration should be measured in seconds, not bytes.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable) not applicable
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes trivial change
• Changes in public API reviewed (if applicable) not applicable

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.6%. Comparing base (5bcaf30) to head (169fa81).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2794   +/-   ##
=====================================
  Coverage   79.6%   79.6%           
=====================================
  Files        124     124           
  Lines      23174   23174           
=====================================
  Hits       18456   18456           
  Misses      4718    4718           

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