add a metric endpoint

[MarcAntoine-Arnaud]

Add a new endpoint to return metrics on the StepFlow
Return:

• number of completed workflows
• number of job errors status
• number of queued
• etc.

It requires to be formatted using the prometheus format.
So the idea is to provide by default the json format, and if mentioned the prometheus format.

GET /metrics -> return the json metrics
GET /metrics?format=json -> return the json metrics
GET /metrics?format=prometheus -> return the prometheus metrics

[nipierre]

Fixed by #86