This prevent `keyword` valued runtime scripts from emitting too many
values or values that take up too much space. Without this you can put
allocate a ton of memory with the script by sticking it into a tight
loop. Painless has some protections against this but:
1. I don't want to rely on them out of sheer paranoia
2. They don't really kick in when the script uses callbacks like we do
   anyway.

Relates to elastic#59332