Add metrics/observability support for AsyncAppenderBase

[guizmaii]

Summary

This PR adds observability/metrics support for the AsyncAppenderBase class, addressing the request in issue #754.

New public methods:

• getTotalAppendedCount() - Returns the total number of events submitted to append()
• getDiscardedByThresholdCount() - Returns events discarded because the queue was nearly full and the event was deemed discardable
• getDiscardedByQueueFullCount() - Returns events discarded when the queue was completely full (neverBlock=true mode)
• getTotalDiscardedCount() - Returns the sum of all discarded events
• getDispatchedCount() - Returns the number of events successfully dispatched to the attached appender
• getFailedDispatchCount() - Returns the number of events that failed during dispatch (uncaught exceptions)
• resetMetrics() - Resets all counters to zero

Design decisions:

• Simple approach: Following @ceki's guidance in the issue discussion, this keeps things simple and doesn't add explicit support for metrics frameworks (Micrometer, Prometheus, etc.). Users can poll these methods and integrate with their monitoring system of choice.
• High-performance counters: Uses LongAdder instead of AtomicLong for better performance under high contention from multiple threads.
• Batch counting: The worker thread uses local counters and batch updates to minimize atomic operations.
• Non-invasive: The core logic remains unchanged; only counters are incremented at appropriate points.

Use cases:

These metrics enable users to:

• Monitor async appender queue depth and throughput
• Detect backpressure issues when events are being discarded
• Alert on excessive event loss
• Track successful vs failed dispatches
• Integrate with monitoring dashboards

Test plan

• Added tests for getTotalAppendedCount()
• Added tests for getDiscardedByThresholdCount()
• Added tests for getDiscardedByQueueFullCount()
• Added tests for getDispatchedCount()
• Added tests for getFailedDispatchCount()
• Added tests for resetMetrics()
• Verified existing tests still pass
• All tests pass: mvn test -pl logback-core -Dtest=AsyncAppenderBaseTest

Closes #754