Stale sessions left over in Redis Shared State

During today's demo we were looking at the flamegraph from #2538 (comment 1593195770):

@reprazent asked a poignant question: Why do we have session keys in ServiceRedis (Shared State), shouldn't those be in ServiceRedisSessions?

Looking at the differential flamegraphs from @msmiley we indeed see that the size of session:lookup:user:gitlab:$NUMBER has remained constant #2538 (comment 1593756454):

This is a strong indicator that these keys are no longer being updated.

Confirming in the code, we can see that there is no expiry set on these values.

Conclusion: These keys have been migrated to ServiceRedisSessions and are safe to be deleted from ServiceRedis (Shared State).