[wip]: optimize blackhole sink performance

[wk989898]

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[gemini-code-assist]

Code Review

This pull request replaces the standard Go channel in the blackhole sink with an UnlimitedChannel and updates the BatchCount method to return the actual channel length. A critical issue was identified in the Run loop where the use of a blocking Get() call within a default block could prevent the sink from responding to context cancellation, hindering graceful shutdowns. It is recommended to use GetWithContext(ctx) instead.

[gemini-code-assist]

The current implementation of the Run loop has a cancellation issue. The select statement with a default block will immediately call s.eventCh.Get(), which is a blocking call. If the context is cancelled while Get() is waiting for an event, the loop will not exit until an event is received or the channel is closed. This prevents the sink from shutting down gracefully when idle. Using GetWithContext(ctx) allows the loop to respond to context cancellation while waiting for events.

		event, ok, err := s.eventCh.GetWithContext(ctx)
		if err != nil {
			return nil
		}
		if !ok {
			log.Info("blackhole sink event channel closed")
			return nil
		}
		event.PostFlush()

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