fix: log sdk no longer enables trace feature

[Copilot]

The logs feature in the OpenTelemetry Rust SDK incorrectly required the trace feature, preventing users from using only logging functionality without bringing in tracing dependencies.

Problem

Previously, enabling only the logs feature would fail to compile because:

1. The SDK's logs feature explicitly depended on opentelemetry/trace in Cargo.toml
2. The logs implementation unconditionally used trace-specific APIs like TraceContextExt

This violated the principle of feature independence and forced users to include tracing dependencies even when only using logs.

Solution

Made trace context retrieval conditional while preserving existing functionality:

• Removed hard dependency: Updated opentelemetry-sdk/Cargo.toml to remove "opentelemetry/trace" from the logs feature
• Conditional trace context: Made span context retrieval conditional with #[cfg(feature = "trace")]
• Preserved compatibility: When both features are enabled, logs automatically pick up trace context from active spans

Behavior Changes

Before: Logs feature required trace feature

# This would fail to compile
opentelemetry_sdk = { features = ["logs"] }  # Error: missing trace feature

After: Logs feature works independently

# Logs only - works without trace dependencies
opentelemetry_sdk = { features = ["logs"] }

# Logs + trace - automatic trace context integration  
opentelemetry_sdk = { features = ["logs", "trace"] }

Verification

• ✅ Logs-only builds and tests pass without trace feature
• ✅ Combined logs+trace features work with automatic trace context pickup
• ✅ Backward compatibility maintained for existing code using both features
• ✅ Created integration tests demonstrating both scenarios

Fixes #3096.

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[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: cijothomas / name: Cijo Thomas (89c36ac, 01b1384, 086ae05, 90e8374, 0556363, e01f857, 45be10a, f3cb22d, 7cc7b79)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.4%. Comparing base (0462369) to head (e01f857).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3102     +/-   ##
=======================================
+ Coverage   80.1%   80.4%   +0.3%     
=======================================
  Files        126     126             
  Lines      21957   22218    +261     
=======================================
+ Hits       17603   17885    +282     
+ Misses      4354    4333     -21     

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[scottgerring]

Great to properly tease the signals apart!