The distributed engine decides when to push down certain operations by
checking if the external labels are still present, i.e. we can push down
a binary operation if its vector matching includes all external labels.
This is great but if you have multiple external labels that are
irrelevant for the partition this is problematic since query authors
must be aware of those irrelevant labels and must incorporate them into
their queries.
This PR attempts to solve that by giving an option to focus on the
labels that are relevant for the partition.

Signed-off-by: Michael Hoffmann <mhoffm@posteo.de>

Co-authored-by: Filip Petkovski <filip.petkovsky@gmail.com>
Signed-off-by: Michael Hoffmann <mhoffm@posteo.de>