As I replied to similar question here.
You need to have a library in your pom that implements the appender for sending data to Loki. If you are using log4j2 then the library I wrote might be useful to you: tjahzi/log4j2-appender at master · tkowalcz/tjahzi · GitHub
If you are using Logback then you have two options, see my post from the first link.
Cheers.