[keepstore] Move legacy metrics to prometheus /metrics endpoint
Currently, keepstore exports some metrics in /status.json. These predate keepstore's prometheus
- BufferPool.BuffersAllocatedCumulative, .BuffersMax, .BuffersInUse
- PullQueue.InProgress, .Queued
- TrashQueue.InProgress, .Queued
- Status.MountPoint, .DeviceNum, .BytesFree, .BytesUsed
- VolumeStats.Errors, .Ops, .CompareOps, .GetOps, .PutOps, .TouchOps, .InBytes, .OutBytes
- InternalStats.* (driver-dependent)
These should be exported as prometheus metrics instead.
There should be a transition period where metrics are exported at both /status.json and /metrics, so ops have an opportunity to migrate their monitoring/dashboard systems.