Add basic prometheus exporter

[lmendes86]

We are recently running Resgate on our platform; thank you for this repo, by the way. However, we still needed basic metrics from it, so we decided to add the possibility of having a Prometheus exporter in Resgate to have insight into how it is operating. We also upgraded to the latest Golang version, but if required, we can remove those changes from this pull request. I hope this helps!