Add disk cache size stats and issue warning if evicted segment has zero length

Note: The component is actually segment-remote but that's not on the list.

The persistent disk cache computes its size internally by adding/subtracting the size of added/purged segments. We would like to be able to see if that computation is correct, by having both the computed size and the effective size on disk in the metrics.

The background for this is that in a few rare instances, the disk cache evicted too many items (down to almost zero) instead of the expected 65% of its max size.

Also we want to make sure that no evictions happen on items that have a length of zero. We're not sure if that happens or not, so the change is to refuse eviction in that case and log a warning instead.