fix: Use doc_cfg instead of doc_auto_cfg

[RasmusRendal]

doc_auto_cfg was removed in rust 1.92.0, and is currently stopping from docs.rs from building (https://docs.rs/crate/rust-telemetry/1.3.0).

Using doc_cfg we get the same feature as doc_auto_cfg used to give us.

Also, hide this behind the docsrs configuration option, to stop absolutely depending on nightly features when building docs.

This commit will not cause docsrs to build rust-telemetry successfully. For this to work, we must update our dependencies (mostly transitive dependencies of OTEL, see #26) to newer versions that do not use doc_auto_cfg either.

But at least, if you depend on rust-telemetry you can now build cargo doc without it crashing.

Testing

I built this on a nightly toolchain with --all-features, and removed the docsrs requirement to get the following:

If I build rust-telemetry with the docsrs configuration option, it will fail because our dependencies still use doc_auto_cfg.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.18%. Comparing base (eac2573) to head (97385db).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #34   +/-   ##
=======================================
  Coverage   74.18%   74.18%           
=======================================
  Files           5        5           
  Lines         275      275           
=======================================
  Hits          204      204           
  Misses         71       71           

Files with missing lines	Coverage Δ
src/lib.rs	88.29% <ø> (ø)

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