Fix: Adjust flush timing for nested generations using outermost_lock_context

[koxudaxi]

Fixes: #176

Overview:
Fix the issue where spans in nested generations are flushed prematurely, causing foreign key constraint failures due to missing parent spans.

Changes:

• Modified the flush logic so that only the outermost generation performs a flush, while inner generations do not trigger a flush.
• Renamed and updated the context manager from manual_flush_context() to outermost_lock_context(). When enabled for the outermost generation (enable_lock=True), outermost_lock_context() acquires the BatchSpanProcessor's condition lock (via "with processor.condition:") to synchronize flush operations. This ensures that flush occurs only once at the end of the outermost generation.
• Updated the generation() decorator to use outermost_lock_context(is_outermost) appropriately, ensuring that flush is executed only for the outermost generation.

Effect:
This change ensures that all spans are sent only after the entire nested generation has completed, preserving the parent-child relationships and avoiding foreign key constraint issues.