fix: add missing stacklevel=2 to warnings.warn() in metrics/

[rtmalikian]

Problem

13 calls to warnings.warn() in monai/metrics/ are missing the stacklevel parameter. Without stacklevel=2, warnings point to the MONAI internal code instead of the user's calling code.

This is the same class of issue as #8929 (losses/ module).

Proposed Fix

Add stacklevel=2 to all 13 warnings.warn() calls in:

• monai/metrics/embedding_collapse.py
• monai/metrics/cumulative_average.py
• monai/metrics/average_precision.py
• monai/metrics/utils.py
• monai/metrics/active_learning_metrics.py
• monai/metrics/confusion_matrix.py
• monai/metrics/rocauc.py