feat(otlp): support INSECURE environment variables for gRPC

[bryantbiggs]

Summary

Add support for 4 INSECURE environment variables per the OTLP exporter specification (SHOULD-level):

• OTEL_EXPORTER_OTLP_INSECURE (generic)
• OTEL_EXPORTER_OTLP_TRACES_INSECURE
• OTEL_EXPORTER_OTLP_METRICS_INSECURE
• OTEL_EXPORTER_OTLP_LOGS_INSECURE

Per the spec, INSECURE only applies to gRPC (not HTTP — HTTP security is determined by URL scheme). There is intentionally no programmatic builder method — this is env-var-only per spec.

Behavior

When an endpoint has no explicit scheme (e.g., collector.example.com:4317):

• INSECURE=true → prepends http:// (plaintext)
• INSECURE=false (default) → prepends https:// and auto-applies ClientTlsConfig::new() when TLS features are enabled

Endpoints with an explicit http:// or https:// scheme are unaffected (scheme detection is case-insensitive).

Signal-specific env var takes precedence over generic; "true" (case-insensitive) = insecure, anything else = secure.

Breaking behavior change

Schemeless endpoints (e.g., collector.example.com:4317) now default to https:// instead of being passed as-is to Channel::from_shared. Users with schemeless endpoints connecting to plaintext gRPC servers should set OTEL_EXPORTER_OTLP_INSECURE=true. Endpoints with an explicit http:// scheme (including the default http://localhost:4317) are unaffected.

Test plan

• cargo clippy -p opentelemetry-otlp --all-targets --all-features -- -D warnings passes
• cargo test -p opentelemetry-otlp --all-features — 117 unit + 2 integration + 21 doc-tests pass
• 5 unit tests for resolve_insecure(): signal overrides generic, default is false, case-insensitive, non-true values return false, falls back to generic
• 4 integration tests for build_channel with INSECURE: schemeless+secure errors without TLS, schemeless+insecure succeeds, schemeless defaults to https, explicit http:// ignores INSECURE env
• All tests use temp_env::with_var_unset for signal-specific var to prevent parallel test interference

Resolves: #774
Resolves: #984

[codecov]

Codecov Report

❌ Patch coverage is 98.05825% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.3%. Comparing base (9650783) to head (e976192).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-otlp/src/exporter/tonic/mod.rs	94.5%	2 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3365   +/-   ##
=====================================
  Coverage   83.2%   83.3%           
=====================================
  Files        128     128           
  Lines      25045   25144   +99     
=====================================
+ Hits       20858   20956   +98     
- Misses      4187    4188    +1     

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