Refactor metric initialization and multiprocessing logic in metric.py

- Changed metric instantiation to accept CPU and GPU indices directly instead of class objects.
- Improved multiprocessing logic to address and fix pickle-related errors.
- Added consistent handling for sync_execution across begin_window and end_window calls for all metrics.
- Centralized bucket range validation and default handling for EnergyHistogram.
- Improved error handling and logging for multiprocessing processes.
- Standardized Prometheus metric labels (e.g., window and index) across Histogram, Counter, and Gauge.
- Updated docstrings for consistency and clarity across all Metric subclasses.