fix(apalis-redis): enable "allow-undeclared-keys" for dragonflydb in stats.lua

[lukapeschke]

#506 introduced a bug when using apalis-redis with dragonfly DB: Since keys get programatically generated, in the for loop counting running jobs, the script does not work with dragonfly, which forbids using undeclared keys by default: https://www.dragonflydb.io/docs/managing-dragonfly/scripting#allowing-undeclared-keys .

With Redis, accessing undeclared keys is allowed, but considered a bad practice: https://redis.io/docs/latest/commands/eval/#:~:text=Important%3A%20to,in%20the%20database

A long-term fix would probably be something like storing a counter of running jobs somewhere, and pass the key of the counter to the script

[lukapeschke]

@geofmureithi does this fix look good to you ? Or do you want me to change anything else ?