I can’t think of any reason why you couldn’t just use log-to-feed "MyKWhFeed" to reset the value to zero, so inserting a “0” without question should just reset the counting.
This should work for any NON-fixed interval feed type, the fixed interval feeds only record the current value if the interval has passed so an unexpected “0” could well get missed.
The schedule processes are not available on emoncms.org so you could try either setting up another node/input to which you send a “0” at the turn of each month (from cron etc) or if self-hosting emoncms use the in-built scheduler to simply add the zeroing processes to the existing input processlist.