[Port to 2.x]: feat: Add DefaultPlanner stats to Engine stats (#27523)

[devanbenz]

Port master-1.x commit f7b4c473be to main-2.x.

Adds planner counters (FindGenerations, Plan, PlanLevel, PlanOptimize) to the engine statistics. 2.x uses prometheus metrics instead of Statistics(); add the counters to the existing compaction metrics in tsdb/engine/tsm1/engine.go.