prometheus.DefBuckets (5ms–10s) are too coarse for Redis operations that
typically complete in under a millisecond.

This commit adds RedisDurationBuckets to v2/metrics/buckets.go and uses them
for the command_duration_seconds histogram in the redis metrics hook.

feat(v2/redis): use sub-millisecond histogram buckets for command duration

See merge request !501

Merged-by: Bob Van Landuyt <bob@gitlab.com>
Approved-by: Bob Van Landuyt <bob@gitlab.com>
Reviewed-by: Bob Van Landuyt <bob@gitlab.com>
Reviewed-by: GitLab Duo <gitlab-duo@gitlab.com>
Co-authored-by: Luke Hollinda <lhollinda@gitlab.com>