chore: can set more etcd endpoint addresses

[daviderli614]

Such as:

helm upgrade --install greptimedb \
  --namespace default \
  --set 'meta.backendStorage.etcd.endpoints[0]=etcd-0.etcd-headless.etcd-cluster.svc.cluster.local:2379' \
  --set 'meta.backendStorage.etcd.endpoints[1]=etcd-1.etcd-headless.etcd-cluster.svc.cluster.local:2379' \
  --set 'meta.backendStorage.etcd.endpoints[2]=etcd-2.etcd-headless.etcd-cluster.svc.cluster.local:2379' \
  greptimedb-cluster