So the pipeline_id is not exposed to the plugin, but since it's part of the thread name, I can extract it from there. I can cut out the pipeline.id from between the [ and ], because the threadname is in the format
[pipeline.id]<plugin.name
For the configuration in log4j2.properties the threadname can be printed with %t, so it will look like this
appender.rolling.layout.pattern = [%d{ISO8601}][%-5p]%t %-.10000m%n
you can strip some of the threadname by using the format modifiers, like %-6t if you use 4 character pipeline id's.