Support custom metrics via YAML configuration

[aviralgarg05]

Summary

Allow users to define custom metrics via YAML configuration instead of Python code.

Tasks

• Define YAML schema for custom metrics (name, formula, thresholds).
• Parse and register metrics from agentunit.yaml or similar config file.
• Support simple formulas referencing built-in metrics (e.g., (faithfulness + tool_success) / 2).
• Document the schema in docs.

Acceptance criteria

• Custom metrics defined in YAML appear in results.
• Invalid YAML produces clear error messages.
• Documentation includes examples.