This attempts to close#3973.
Handles cases where the length of the input vector to an aggregate topk
/ bottomk function is less than the K paramater. The change updates
Prometheus to allocate a result vector the same length as the input
vector in these cases.
Previously Prometheus would out-of-memory panic for large K values. This
change makes that unlikely unless the size of the input vector is
equally large.
Signed-off-by: David King <dave@davbo.org>
This offers new semantics in allowing on() for matching
two single-element vectors with no known common labels.
Previosuly this was often done using on(dummy).
This also allows making it explict that you meant
to do an aggregation without labels via by().
Fixes#1597.
This has the advantage that the user doesn't need
to list all labels they want to keep (as with "by")
but without having to worry about inconsistent labels
as when there's only one time series (as with "keeping_common").
Almost all aggregation should use this rather than the existing
two options as it's much less error prone and easier to maintain
due to not having to always add in "job" plus whatever other common
job-level labels you have like "region".
