refactor: Replace OnceCell with std::sync::OnceLock

[FalkWoldmann]

Fixes #
Design discussion issue (if applicable) #

Changes

Replaces once_cell::sync::OnceCell with itsstd equivalent. We cannot yet remove the whole once_cell crate, since we still use Lazy and its std counterpart LazyLock requires rust 1.80

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]

• ✅login: FalkWoldmann / (1ea3b94)
• ✅login: FalkWoldmann / (1ea3b94, 99c05ec)

The committers listed above are authorized under a signed CLA.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.0%. Comparing base (86e842c) to head (99c05ec).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2899   +/-   ##
=====================================
  Coverage   81.0%   81.0%           
=====================================
  Files        125     125           
  Lines      23945   23945           
=====================================
  Hits       19414   19414           
  Misses      4531    4531           

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[lalitb]

Changes look good, but I believe this crate won't accept changes any longer as mentioned here - https://github.com/open-telemetry/opentelemetry-rust/tree/main/opentelemetry-prometheus

-clip
Warning: This crate is no longer recommended for use.

Development of the Prometheus exporter has been discontinued. See the related #2451. This crate depends on the unmaintained protobuf crate and has unresolved security vulnerabilities. Version 0.29 will be the final release.

For Prometheus integration, we strongly recommend using the OTLP exporter instead. Prometheus natively supports OTLP, providing a more stable and actively maintained solution.

[FalkWoldmann]

Ah, I missed this comment. Thanks for the feedback!