Fast truncate has no Grafana representation, so a truncate problem on a live node has to be diagnosed from raw statistics.

Add panels covering:

• truncate rate, split by leader and follower
• truncate list length and its high water mark
• search cost: calls, entries walked, and the resulting entries walked per call
• GC activity: runs and entries removed
• truncate write conflict rate

Depends on the statistics added under the truncate list statistics ticket.