feat: prometheus monitoring

[manu0466]

Description

This PR allows to expose a /metrics endpoint that can be used to monitor a parser trough prometheus.

The monitoring can be enabled trough the config file by adding a new monitoring section like this:

monitoring:
  enabled: true
  port: 2112

Checklist

• Targeted PR against correct branch.
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Wrote unit tests.
• Re-reviewed Files changed in the Github PR explorer.

[RiccardoM]

Can you please fix the linting errors?

[RiccardoM]

Wouldn't this case an error if config.Monitoring is null (i.e. it was not sent inside the config? I think we should just return a nil Prometheus instead. We could just pass config.Monitoring to the NewServer method and return a nil instance if the monitoring is either disabled or the config is not set at all. Then, inside the main we just call .Start with a nil check and if the server is nil then we do nothing. What do you think?

[manu0466]

Yes, I was forcing the definition of the monitoring section inside the config. We can make it optional, as you suggest, to maintain backward compatibility.