refactor(otlp): deprecate tls feature in favor of explicit tls-ring and tls-aws-lc

[noahdormann]

The generic tls feature currently enables tonic/tls-ring, which implicitly selects the ring rustls crypto provider.
This makes it difficult or impossible for downstream users to use alternative rustls providers such as aws-lc, and can lead to feature unification conflicts when combined with other TLS-enabled dependencies.

This PR deprecates the generic tls feature and introduces explicit tls-ring and tls-aws-lc features, exposing both underlying tonic TLS options and allowing consumers to consistently select a single rustls crypto provider.

Motivation

Rustls 0.23 requires a single, unambiguous crypto provider (ring or aws-lc).
Because Cargo features are additive, the existing tls feature could silently force ring even when users explicitly opted into aws-lc elsewhere, resulting in runtime panics during TLS initialization.

Providing explicit TLS feature flags avoids this class of failure and aligns with tonic’s provider model.

Changes

• Deprecate the existing tls feature
• Introduce:
  • tls-ring → enables tonic/tls-ring
  • tls-aws-lc → enables tonic/tls-aws-lc
• Update documentation to recommend selecting exactly one TLS feature

Migration

Users currently relying on tls should switch to one of:

features = ["tls-ring"]

or

features = ["tls-aws-lc"]

The deprecated tls feature remains temporarily for compatibility but should not be used in new configurations.

Caveat

The feature tls-roots and tls-webpki-roots can no longer explicitly enable tls. This could lead to issues with users that relied on this impicit feature map.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: noahdormann / name: Noah Dormann (b27be42)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.4%. Comparing base (e5f602e) to head (b27be42).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3323   +/-   ##
=====================================
  Coverage   81.4%   81.4%           
=====================================
  Files        128     128           
  Lines      23742   23742           
=====================================
+ Hits       19328   19332    +4     
+ Misses      4414    4410    -4     

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[lalitb]

LGTM, with updated changelog

[noahdormann]

@lalitb Thank you for your review. I have amended the commit with the updated changelog.