Remove obsolete comment. Reinit controller stats on engine open

Signed-off-by: Rohit Nayak <rohit@planetscale.com>

go/vt/vttablet/tabletmanager/vdiff/engine.go

@@ -146,6 +146,8 @@ func (vde *Engine) openLocked(ctx context.Context) error {
 		vde.resetControllers()
 	}
 
+	globalStats.initControllerStats()
+
 	// At this point the tablet has no controllers running. So
 	// we want to start any VDiffs that have not been explicitly
 	// stopped or otherwise finished.
@@ -222,7 +224,6 @@ func (vde *Engine) addController(row sqltypes.RowNamedValues, options *tabletman
 	defer globalStats.mu.Unlock()
 	globalStats.controllers[ct.id] = ct
 
-	// run() can start a vdiff in pending/started state, so we should init the stats first before starting it.
 	controllerCtx, cancel := context.WithCancel(vde.ctx)
 	ct.cancel = cancel
 	go ct.run(controllerCtx)

go/vt/vttablet/tabletmanager/vdiff/stats.go

@@ -44,12 +44,18 @@ type vdiffStats struct {
 	RowsDiffedCount     *stats.Counter
 }
 
+func (vds *vdiffStats) initControllerStats() {
+	vds.mu.Lock()
+	defer vds.mu.Unlock()
+	vds.controllers = make(map[int64]*controller)
+}
+
 func (vds *vdiffStats) register() {
 	globalStats.Count = stats.NewGauge("", "")
 	globalStats.ErrorCount = stats.NewCounter("", "")
 	globalStats.RestartedTableDiffs = stats.NewCountersWithSingleLabel("", "", "Table")
 	globalStats.RowsDiffedCount = stats.NewCounter("", "")
-	globalStats.controllers = make(map[int64]*controller)
+	globalStats.initControllerStats()
 
 	stats.NewGaugeFunc("VDiffCount", "Number of current vdiffs", vds.numControllers)