Shard allocation based on shard size

[DavidTurner]

@coudenysj

The problem we see is that in busy days, the apache filebeat logs grow a lot, so we need to have some buffer, but we want it as small as possible.

I see - that is a better reason for relocating shards

The usual solution is to set the gap between the low and high watermarks to be larger than the typical size of the day's indices on each node, and the gap between the high and flood-stage watermarks to be large enough to allow time to mitigate any overage before disks fill up. This largely works in practice, but it's not completely ideal.