Problem

When MeterProvider::Shutdown() is called manually, the destructor still calls Shutdown() again, which causes MeterContext to emit a warning:

[MeterContext::Shutdown] Shutdown can be invoked only once.

This creates unnecessary noise in logs when users properly shut down their meter providers manually before destruction.

Root Cause

The MeterProvider destructor unconditionally calls context_->Shutdown() without checking if shutdown has already been performed:

MeterProvider::~MeterProvider()
{
  if (context_)
  {
    context_->Shutdown();  // Always calls shutdown
  }
}

Solution

This PR implements the same pattern used by other components in the codebase like BatchSpanProcessor and SimpleLogRecordProcessor:

1. Added IsShutdown() method to MeterContext - Allows checking shutdown state without side effects
2. Added atomic<bool> is_shutdown_ member - Tracks shutdown state independently of the shutdown latch
3. Updated MeterProvider destructor - Only calls Shutdown() if not already shut down
4. Preserved existing behavior - Manual duplicate shutdown calls still emit warnings as expected

Changes

Before:

MeterProvider::~MeterProvider()
{
  if (context_)
  {
    context_->Shutdown();  // Always warns if shutdown was called manually
  }
}

After:

MeterProvider::~MeterProvider()
{
  if (context_ && !context_->IsShutdown())
  {
    context_->Shutdown();  // Only shutdown if not already done
  }
}

Testing

• ✅ Existing tests pass without destructor warnings
• ✅ New ShutdownTwice test validates the fix
• ✅ Manual duplicate shutdown calls still warn appropriately
• ✅ All metrics tests continue to pass

Before Fix

[Warning] File: .../meter_context.cc:178 [MeterContext::Shutdown] Shutdown can be invoked only once.

After Fix

No warnings from destructor when Shutdown() was called manually.

Fixes #3511.

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