Fix OTLP MetricExporter deadlock by switching to tokio::Mutex

[Copilot]

The OTLP Tonic MetricExporter was using std::sync::Mutex to protect the inner client, but the lock was being held across await points in the export method. This could cause deadlocks because std::Mutex is not async-safe and blocks the entire thread when contended.

Problem

// Before - problematic pattern
let (mut client, metadata, extensions) = self
    .inner
    .lock()  // std::Mutex lock acquired
    .and_then(|mut inner| {
        // ... interceptor work that might block ...
        Ok((inner.client.clone(), m, e))
    })?;  // Lock held until here

// Then immediately:
client.export(...).await;  // Async operation while lock might still be conceptually held

The issue occurs when multiple concurrent export operations compete for the mutex, especially if the interceptor logic involves any blocking operations.

Solution

Switch to tokio::sync::Mutex and use the async lock pattern already established by the LogsExporter:

// After - async-safe pattern  
let (mut client, metadata, extensions) = match self.inner.lock().await.as_mut() {
    Some(inner) => {
        // ... interceptor work ...
        (inner.client.clone(), m, e)
    }
    None => return Err(OTelSdkError::InternalFailure("exporter is already shut down".into())),
};

client.export(...).await;  // Safe async operation

Changes

• Changed import from std::sync::Mutex to tokio::sync::Mutex
• Updated export method to use .lock().await instead of .lock()
• Simplified shutdown method with TODO comment about async limitations (consistent with LogsExporter)

Verification

• All existing tests continue to pass
• Build succeeds without warnings
• Pattern matches the established LogsExporter implementation
• HTTP client was already correctly implemented and unaffected

Fixes #2802.

---

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.1%. Comparing base (0462369) to head (401f741).

Files with missing lines	Patch %	Lines
opentelemetry-otlp/src/exporter/tonic/metrics.rs	0.0%	14 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3107   +/-   ##
=====================================
  Coverage   80.1%   80.1%           
=====================================
  Files        126     126           
  Lines      21957   21948    -9     
=====================================
- Hits       17603   17599    -4     
+ Misses      4354    4349    -5     

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