feat: adding custom metrics from API service

[ssheng]

There is an increasing demand from the community for adding custom metrics to the API service. BentoML supports basic service level metrics out-of-box, including request duration, in-progress, and count, using a prometheus_client. However, it is not as straight forward for users to create new metrics. An ideal API allows users to define metrics during initialization and update metrics upon a certain event (e.g. request, response, error).