With classic logging you'd:

logger.debug("some var: %s", the_var)

so that a perhaps costly conversion or interpolation isn't payed if the log level isn't debug or trace.
How would we do the same with structured logging?

logger.debug("some var: %s", the_var, extras={})

could work for the message but what if I need it inside another field?

logger.debug({"key1": "%s", "key2": "%s"}, var1, var2)

This looks nasty.

Or is there already something in place I've missed?