Provide prometheus metrics #153

We should provide metrics about the Gitploy application that users can manage Gitploy themselves. In the issue, we'll consider 1) which metrics are being and 2) how to implement them.

Metrics

• The response code of the server
• The count of repositories
• The count of deployment of repository for each environment (e.g. dev, production), respectively
• The count of user
• The limit of user