Operators could see attach-ready and capacity-pressure events but not how
full the configured disk (or memory) tier was without reading filesystems.
Expose size/capacity gauges and disk_cache_full_ratio next to disk_tier_ready.
Capacity is a config read, so it stays available while attach is pending.
/metrics was Prometheus-ish name/value text with a custom banner and
text/plain Content-Type, so strict scrapers could fail.
Emit Prometheus text 0.0.4 (# HELP, # TYPE, counter|gauge) with stable
metric names, and set Content-Type to text/plain; version=0.0.4; charset=utf-8.
When the disk (or memory) tier is at cap or the volume returns ENOSPC,
ops currently look like random misses with no clear "we are dropping
data." Count those events as capacity_pressure_events on /metrics and
log tier plus reason so operators can tell capacity pressure from a
cold cache, without changing the existing evictions counter.
Link: #36
pike
ash