Hey Alex,
sorry for the late response. Do you mean you want to filter just for specific documents before calculating sum of that account? In this case you just want to use the q(uery) parameter:
.es(index='billing', split=account.from.number:1, metric=sum:respond.cost, q='request.basic:"some string"')
The query is of the same format, than any query you can put in the query bar on top of dashboard/visualize/discover. So it should match that format.
Cheers,
Tim