ci: split lint into parallel jobs and remove redundant checks

[bryantbiggs]

Summary

The CI lint job is the most frequent source of PR failures. After investigating ~20 recent failures, every single one was a legitimate clippy error (dead code, unused variables, etc.) caught by cargo hack --each-feature — not resource exhaustion or flakiness.

The root cause: PRs introduce code that compiles fine with --all-features but triggers warnings when individual features are tested in isolation (e.g., a function only used with grpc-tonic enabled but missing #[cfg(feature = "grpc-tonic")]). Contributors don't know to run cargo hack locally, so they only discover the issue after pushing.

This PR improves the situation by:

• Splitting lint into two parallel CI jobs for faster feedback:

  • lint: format, workspace clippy, and multi-feature combo checks (~1-2 min)
  • lint-feature-matrix: per-feature clippy via cargo hack --each-feature (~5-6 min)

  Previously all ~150 clippy invocations ran sequentially in a single job. Now the quick checks give immediate feedback while the thorough feature matrix runs in parallel.

• Removing 6 redundant cargo_feature checks from lint.sh that are already covered by cargo hack --each-feature:

  • opentelemetry-otlp "default", "http-proto", "metrics" (single-feature, already tested individually by cargo hack)
  • opentelemetry-jaeger-propagator "default" (same)
  • opentelemetry-proto "default", "full" (same)

• Updating CONTRIBUTING.md to document the cargo hack lint command so contributors can catch per-feature issues locally before pushing.

• Updating scripts/precommit.sh to include the feature matrix check.

Test plan

• Verify lint job passes (format + workspace clippy + multi-feature combos)
• Verify lint-feature-matrix job passes (cargo hack per-feature checks)
• Confirm both jobs run in parallel, not sequentially
• Confirm no regressions in lint coverage (all multi-feature combos still checked)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.2%. Comparing base (09b85b5) to head (10157b4).

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3366     +/-   ##
=======================================
- Coverage   82.2%   82.2%   -0.1%     
=======================================
  Files        128     128             
  Lines      24626   24626             
=======================================
- Hits       20267   20266      -1     
- Misses      4359    4360      +1     

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