docs(logs): polish design doc and fill perf numbers

[cijothomas]

Polish pass over docs/design/logs.md:

• Minor wording/typo fixes in the API and LogProcessor sections.
• Fill in Perf test - benchmarks and Perf test - stress test with numbers and links to the actual benchmarks (opentelemetry-appender-tracing/benches/logs.rs) and the stress harness (stress/src/logs.rs), and add a short framing paragraph to the Performance section.
• Internal logs section: replace the two // TODO placeholders. The severity-selection TODO is dropped (out of scope for a Logs-signal design doc). The "OTLP can solve this" TODO is replaced with a short paragraph noting the open suppression gap for hyper/tonic and linking the tracking issue (Fix telemetry-induced-telemetry in OTLP Exporter #2877).

No code or behavior changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.2%. Comparing base (0e78170) to head (a9c2c8c).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3566   +/-   ##
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  Coverage   83.2%   83.2%           
=====================================
  Files        130     130           
  Lines      28246   28246           
=====================================
  Hits       23506   23506           
  Misses      4740    4740           

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[github-actions]

Thank you for your contribution! This PR has been automatically marked as stale because it has not had activity in the last 14 days. This may be due to a delay in review on our side or awaiting a response from you; either is fine, and we appreciate your patience.

It will be closed in 14 days if no further activity occurs. Pushing a new commit or leaving a comment will remove the stale label and keep the PR open.

[cijothomas]

@utpilla @lalitb PTAL.