Document initial api

The plan is to document the initial api methods and use the PR to get everyone's feedback on how the api should look before starting on the implementation.
This should probably be `/metrics/` if we are planning on including dashboards as part of the api. I think it might look weird though: `/api/metrics/metrics/?m=...` and `/api/metrics/dashboards/...`. It kind of feels more natural to have this be a separate api. Happy to discuss this, but its probably something we need to decide now to avoid breaking the api.