fix(callbacks): log metric callbacks

[sarvesh1327]

Summary

• Replaces the default metric callback print() calls with module logger calls.
• Keeps prediction timing and active-request messages visible, while allowing logging handlers/configuration to capture and route them.
• Updates callback tests to assert log records for the direct callback paths and preserves the integration stdout assertion for the worker-process path.

Test Plan

• uv run python -m pytest tests/unit/test_callbacks.py -q
• uv run ruff check src/litserve/callbacks/defaults/metric_callback.py tests/unit/test_callbacks.py
• uv run python -m py_compile src/litserve/callbacks/defaults/metric_callback.py tests/unit/test_callbacks.py
• git diff --check

Refs #696