Replace once_cell::Lazy with std::sync::OnceLock for global Initialization in OpenTelemetry API crate

[lalitb]

Changes

• std::sync::OnceLock was stabilized in Rust 1.70.0, providing equivalent functionality to once_cell::Lazy directly in the standard library. This change reduces the dependency on an external crate (once_cell) for a feature now natively supported.
• Both once_cell::Lazy and std::sync::OnceLock guarantee thread-safe, one-time initialization of global variables. The replacement does not alter the functionality or behavior of the code.
• Also unrelated change - pin-project-lite, futures-sink, futures-core, thiserror are made optional dependencies, and only used when trace feature is enabled. With this, opentelemetry crate doesn't use any external crate for logs and metrics.

Example of change for TracerProvider:
Before:

use once_cell::sync::Lazy;
use std::sync::{Arc, RwLock};

static GLOBAL_TRACER_PROVIDER: Lazy<RwLock<GlobalTracerProvider>> =
    Lazy::new(|| RwLock::new(GlobalTracerProvider::new(NoopTracerProvider::new())));

After

use std::sync::{OnceLock, RwLock};

static GLOBAL_TRACER_PROVIDER: OnceLock<RwLock<GlobalTracerProvider>> = OnceLock::new();

#[inline]
fn global_tracer_provider() -> &'static RwLock<GlobalTracerProvider> {
    GLOBAL_TRACER_PROVIDER
        .get_or_init(|| RwLock::new(GlobalTracerProvider::new(NoopTracerProvider::new())))
}

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)

[codecov]

Codecov Report

Attention: Patch coverage is 58.62069% with 12 lines in your changes missing coverage. Please review.

Project coverage is 79.4%. Comparing base (3d352d8) to head (00a9626).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry/src/global/propagation.rs	0.0%	12 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2326     +/-   ##
=======================================
- Coverage   79.5%   79.4%   -0.1%     
=======================================
  Files        123     123             
  Lines      21258   21271     +13     
=======================================
+ Hits       16905   16910      +5     
- Misses      4353    4361      +8     

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[cijothomas]

Thanks for driving to keep external dependencies minimal!

Lets merge after we get one more eyes to review.

[utpilla]

nit: For each of these methods, we could use a comment similar to what we have for Baggage:

/// Returns the default baggage, ensuring it is initialized only once.

So, something like:

/// Returns the global TextMapPropagator ensuring it is initialized only once.

[utpilla]

Also unrelated change - pin-project-lite, futures-sink, futures-core, thiserror are made optional dependencies, and only used when trace feature is enabled.

We have trace added as a default feature. Do we want to change this to avoid adding these dependencies for the default build?

[utpilla]

Left some non-blocking suggestions.

[lalitb]

Also unrelated change - pin-project-lite, futures-sink, futures-core, thiserror are made optional dependencies, and only used when trace feature is enabled.

We have trace added as a default feature. Do we want to change this to avoid adding these dependencies for the default build?

Yes we can discuss this separate to this PR, as it will affect the trace consumers relying on the default build.

[cijothomas]

Also unrelated change - pin-project-lite, futures-sink, futures-core, thiserror are made optional dependencies, and only used when trace feature is enabled.

We have trace added as a default feature. Do we want to change this to avoid adding these dependencies for the default build?

Yes we can discuss this separate to this PR, as it will affect the trace consumers relying on the default build.

For 1.0, it'd better to remove trace from default features, as we are not in a position to declare traces as stable. Lets track this separately.