fix: Make NoopMeterProvider public to allow external usage

[Copilot]

This PR restores public access to NoopMeterProvider which was made private in PR #2191. The NoopMeterProvider is useful for applications that want to conditionally disable telemetry at compile time without changing their instrumentation code.

Changes

• Made the noop module public in opentelemetry/src/metrics/mod.rs
• Changed NoopMeterProvider struct visibility from pub(crate) to pub
• Changed NoopMeterProvider::new() method visibility from pub(crate) to pub
• Added NoopMeterProvider to the public exports in the metrics module

Usage

Users can now directly use NoopMeterProvider for conditional telemetry:

use opentelemetry::metrics::{MeterProvider, NoopMeterProvider};

// Create a no-op meter provider for conditional telemetry
let provider = NoopMeterProvider::new();
let meter = provider.meter("my_app");

// All instruments created from this meter will be no-ops
let counter = meter.u64_counter("requests").build();
counter.add(1, &[]); // This operation is a no-op

This allows applications to use feature flags or environment variables to conditionally disable telemetry without requiring code changes or copying the NoopMeterProvider implementation.

Fixes #2444.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.4%. Comparing base (86dfd4b) to head (ba26861).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3111   +/-   ##
=====================================
  Coverage   80.4%   80.4%           
=====================================
  Files        126     126           
  Lines      22237   22237           
=====================================
  Hits       17898   17898           
  Misses      4339    4339           

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