Adds DisposeAsync to IDogStatsd Interface for Tracer Usage

[link04]

Summary

Exposes DisposeAsync() on the public IDogStatsd interface, allowing callers to await disposal and avoid sync-over-async patterns.

Changes

• IDogStatsd now has DisposeAsync()
• DogStatsdService implements it
• Added a couple tests for the new method

Motivation

With DisposeAsync(), callers can now do await dogstatsd.DisposeAsync(); instead of blocking with dogstatsd.Dispose();.

Testing

• DisposeAsync_CanBeCalledMultipleTimes - verifies concurrent dispose calls don't deadlock
• DisposeAsync_Completes_WithInFlightMetrics - verifies disposal completes even with ongoing metric traffic

Manual benchmark CompareSyncVsAsync_UnderStarvation (run with [Explicit] test) shows ~3x faster disposal under thread pool pressure(mostly added to showcase a difference).

=== THREAD POOL STARVATION BENCHMARK ===
Thread pool: min=2, max=4 (severely constrained)
Blocking tasks: 10 (exceeds pool capacity)

SYNC  Dispose: 144ms
ASYNC Dispose: 44ms  ← 3x faster under starvation

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