test: fix and extend benchmarks

[jpnurmi]

• Fix logs benchmark before the upcoming optimizations ([WIP] perf: read-write-locked scope data #2042)
  • 32×100 attempts exceeded the 10×100 buffer capacity, so quick discards made results artificially good. Reduce iterations per thread to 32, bringing the peak to 1024 against a 1000-item capacity.
• Report enqueue failures and count only successful enqueues as processed items for logs and metrics.
• Use real time to measure elapsed time across concurrent threads.
• Add distribution metrics benchmarks with 1, 8, 16, and 32 threads.
• Add contexts benchmarks across all four backends.
• Split scope benchmarks into dedicated tags and breadcrumbs benchmarks.

Ref: #1862

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.32%. Comparing base (0a31c85) to head (96a20da).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2085      +/-   ##
==========================================
- Coverage   74.37%   74.32%   -0.06%     
==========================================
  Files         103      103              
  Lines       26817    26817              
  Branches     4881     4881              
==========================================
- Hits        19946    19931      -15     
- Misses       5531     5546      +15     
  Partials     1340     1340              

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