kv_vb_queue_age_seconds metric can underflow

This metric is calculated based on counters from the flusher in KV. When a mutation comes in, we increment dirtyQueueSize by 1 and dirtyQueueAge by the queue time of the mutation. When the mutation is written to disk, we decrement these by the same amounts.

Metrics collection runs on a separate thread from the FlusherTask (which writes to disk), so a metrics scrape can observe an incremented dirtyQueueAge and non-incremented dirtyQueueSize (same in the decrement case).

The kv_vb_queue_age_seconds is computed as:

now() * dirtyQueueSize - dirtyQueueAge

Where dirtyQueueAge is loaded from memory first, we can see a dirtyQueueAge that is small at the end of the flush batch and a dirtyQueueSize which is larger if new mutations just came in, resulting in underflow which wraps around.