Perform full sample of nested keys for memkeys

By default, the redis 'memory usage' command (used by --memkeys) only samples 5 elements from nested keys (like lists, sets, zsets etc) and assumes they are a representative average. Empirically, this is not true for gitlab.com's usage, particularly in sidekiq. See https://gitlab.com/gitlab-com/gl-infra/production/-/issues/3404#note_494627089

This alters the invocation of memkeys to request that all nested items are sampled. This will increase runtime, but empirically by well less than 10%