Add slab to mem plugin (#3518)

plugins/inputs/system/memory.go

@@ -32,6 +32,7 @@ func (s *MemStats) Gather(acc telegraf.Accumulator) error {
 		"buffered":          vm.Buffers,
 		"active":            vm.Active,
 		"inactive":          vm.Inactive,
+		"slab":              vm.Slab,
 		"used_percent":      100 * float64(vm.Used) / float64(vm.Total),
 		"available_percent": 100 * float64(vm.Available) / float64(vm.Total),
 	}