Add cron-like expression fro scrappers

[bcecchinato]

Hi !

We have a use case on which we multiple servers that are running beamium, and we want to avoid having them to scrap a metric at the time. Would it be possible to add a cron-like expression to define when to scrap the metric, this would allow us to do for instance :

• On server 1 : 5/20 0 0 ? * * *
• On server 2 : 15/20 0 0 ? * * *
• On server 3 : 25/20 0 0 ? * * *

That would make beamium more flexible. If you need more details you can jabb me :)