feat: haproxy periodic query - expand and sum by backend

Applying Victor's suggestion in https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/15437#note_900135090.

First introduced in !4481 (merged).

New Thanos query:

sum by (backend)(increase(haproxy_backend_bytes_out_total{backend!~"canary.*|camoproxy|429.*",device!="lo", env="gprd"}[1d]))