Missing documentation for Metadata/Tags

While writing a backend for SLF4J, I stumbled across the Flogger concept of Metadata and Tags, which seem like they would map very well to SLF4J's concept of [MDC](https://logback.qos.ch/manual/mdc.html). However, while I can see that one receives the Metadata and Tags in various places, my search has not turned up any way to actually **set** them.

It would be very helpful if someone could drop a few lines of code showing how to set a simple Tag to point me in the right direction so I could implement the parsing/delegating of it in the SLF4J backend.