Preserve SLI selectors on service rate queries
Our rate queries building logic sometimes inserts additional selectors.
For example, when building request rates based on an SLI's significant
labels
(https://gitlab.com/gitlab-com/runbooks/-/blob/7360e4c30c80511c32d32a4c2ce3d0939ef7abb7/libsonnet/gitlab-dashboards/metrics_catalog_dashboards.libsonnet#L346)
we add a ne: ''
selector
(https://gitlab.com/gitlab-com/runbooks/-/blob/7360e4c30c80511c32d32a4c2ce3d0939ef7abb7/libsonnet/gitlab-dashboards/metrics_catalog_dashboards.libsonnet#L463)
for the corresponding label. If the SLI definition also specifies a
selector for that label, it'll be overwritten by the one added by our
query generation code. Let's instead assume that the SLI selectors are
more significant and give them preference
Detected during !5915 (merged)