Add Loki target

[AndiDog]

What does this do?

Loki targets require the "datasource": {"type": "loki", "uid": "[...]"} field split in order to work in Grafana – as tested in Grafana Cloud which would give me cryptic error messages otherwise.

Why is it a good idea?

Loki is a great way to quickly put together custom dashboards if only (parseable) logs are available for an application, but not metrics. There was no previous example how to use grafanalib for Loki queries.

Questions

I added a basic test. Not sure if I should add an example or more documentation.

Also, I made the LokiTarget a standalone class, not based on Target, since the "base fields" in Target are not desired (e.g. adding the field query or others could lead to errors, I think).

[JamesGibo]

Thanks for adding this, I will merge this PR, but if you do find time to update the docs to say it supports Loki and provide an example, I think that would be really helpful.