This fixes an issue where it was impossible to specify a collector that
was available but not selected by default.
Instead of checking whether chosen collectors are valid at flag parse
time, this moves the check into the builder, where we can reference it
against the availableStores in the builder. As a bonus, the error
message also prints out a list of available collectors:
```
kube-state-metrics --collectors non-existent-collector
I0618 15:23:34.517532 50719 main.go:88] Using collectors non-existent-collector
F0618 15:23:34.519132 50719 main.go:90] Error: collector non-existent-collector does not exist. Available collectors: persistentvolumeclaims,configmaps,limitranges,nodes,namespaces,persistentvolumes,pods,replicasets,services,cronjobs,deployments,ingresses,horizontalpodautoscalers,jobs,poddisruptionbudgets,secrets,certificatesigningrequests,daemonsets,endpoints,storageclasses,replicationcontrollers,resourcequotas,statefulsets
```
Instead of iterating a map of enabled collectors, iterate a sorted
slice to achieve determinism across scrapes.
Having a consistent order in the metrics output enables Proemetheus to
apply optimizations during metric parsing and ingestion.
蓝宝石的傻话
Catherine Fang
John
Min Wang
xubin5
Mikulas (Mike) Dite
Lili Cosic
Joel Whittaker-Smith
Alexander-Carpenter
Ben Wells
Miles Bryant
reetasingh
tariqibrahim
Max Leonard Inden
Frederic Branczyk
Andy Xie