[release-19.0] Fix query plan cache misses metric (#16562) (#16627)

Signed-off-by: shanth96 <shanth.sathiyaseelan@shopify.com>
Co-authored-by: vitess-bot[bot] <108069721+vitess-bot[bot]@users.noreply.github.com>

go/vt/vtgate/executor.go

@@ -202,7 +202,7 @@ 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)