Merge branch '469595-fix-mean-median-labels' into 'master'

Use correct label for 50th percentile in charts

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/184044



Merged-by: Miranda Fluharty <mfluharty@gitlab.com>
Approved-by: Miranda Fluharty <mfluharty@gitlab.com>
Co-authored-by: Miguel Rincon <mrincon@gitlab.com>