Example with metric list and custom metric

Documentation should contains page devoted to metrics. It should consist of
* list of metrics for optimization
* code shows how to pass custom metric for the API

The issue names no documentation file or entry point, so first locate the repository's documentation structure and the API documentation for custom metrics. Add a metrics page covering optimization metrics and a Python example showing how to pass a custom metric; done means both requested topics are documented.

Scritto dal modello di indicizzazione a partire dal testo della issue.