Fix query plan cache misses metric (vitessio#16562)

Signed-off-by: shanth96 <shanth.sathiyaseelan@shopify.com>

go/vt/vtgate/executor.go

@@ -208,7 +208,7 @@ func NewExecutor(
 			return e.plans.Metrics.Hits()
 		})
 		stats.NewCounterFunc("QueryPlanCacheMisses", "Query plan cache misses", func() int64 {
-			return e.plans.Metrics.Hits()
+			return e.plans.Metrics.Misses()
 		})
 		servenv.HTTPHandle(pathQueryPlans, e)
 		servenv.HTTPHandle(pathScatterStats, e)