feat(stress): add bound counter/histogram stress tests

[cijothomas]

Adds two new stress test bins for the bound instruments API (gated behind experimental_metrics_bound_instruments):

• metrics_counter_bound — single BoundCounter created at startup; hot loop calls add(1).
• metrics_histogram_bound — single BoundHistogram created at startup; hot loop calls record(1).

This complements the existing bound-instrument benchmark in opentelemetry-sdk/benches/bound_instruments.rs by exercising the same hot path under sustained multi-threaded load.

Also refreshes the throughput numbers in the headers of all metric/trace/log stress tests with results captured on Apple M4 Pro (macOS 26.4.1, 14 cores), since none of the existing tests had numbers recorded for this hardware.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.8%. Comparing base (8db5a12) to head (71e1197).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3516   +/-   ##
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  Coverage   82.8%   82.8%           
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  Files        130     130           
  Lines      27289   27289           
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  Hits       22622   22622           
  Misses      4667    4667           

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