core/consensus: fix incorrect metric label (#2443)

Fixes the consensus duration metric label to duty type, which fixes the high cardinality and memory issue.

category: bug
ticket: #2438

core/consensus/component.go

@@ -312,7 +312,7 @@ func (c *Component) propose(ctx context.Context, duty core.Duty, value proto.Mes
 		case decidedAt := <-inst.decidedAtCh:
 			timerType := c.timerFunc(duty).Type()
 			duration := decidedAt.Sub(proposedAt)
-			consensusDuration.WithLabelValues(duty.String(), string(timerType)).Observe(duration.Seconds())
+			consensusDuration.WithLabelValues(duty.Type.String(), string(timerType)).Observe(duration.Seconds())
 		default:
 		}
 	}()