Today the `DiskThresholdDecider` attempts to account for already-relocating
shards when deciding how to allocate or relocate a shard. Its goal is to stop
relocating shards onto a node before that node exceeds the low watermark, and
to stop relocating shards away from a node as soon as the node drops below the
high watermark.
The decider handles multiple data paths by only accounting for relocating
shards that affect the appropriate data path. However, this mechanism does not
correctly account for _new_ relocating shards, which are unwittingly ignored.
This means that we may evict far too many shards from a node above the high
watermark, and may relocate far too many shards onto a node causing it to blow
right past the low watermark and potentially other watermarks too.
There are in fact two distinct issues that this PR fixes. New incoming shards
have an unknown data path until the `ClusterInfoService` refreshes its
statistics. New outgoing shards have a known data path, but we fail to account
for the change of the corresponding `ShardRouting` from `STARTED` to
`RELOCATING`, meaning that we fail to find the correct data path and treat the
path as unknown here too.
This PR also reworks the `MockDiskUsagesIT` test to avoid using fake data paths
for all shards. With the changes here, the data paths are handled in tests as
they are in production, except that their sizes are fake.
Fixes#45177
d340530a47