etcdctl unsetting ETCDCTL_API in lock exec

[devinrsmith]

etcdctl lock <lockname> exec appears to be unsetting the environment variable ETCDCTL_API for the executed command. (And it might be unsetting other ETCDCTL_X variables, I haven't checked...)

For example:

13:59 $ MY_API=3 ETCDCTL_API=3 etcdctl lock /locks env | grep API
MY_API=3
✔ ~

Only MY_API shows up in the output.

13:59 $ etcd --version
etcd Version: 3.3.12
Git SHA: GitNotFound
Go Version: go1.11.5
Go OS/Arch: darwin/amd64
✔ ~

[spzala]

Investigating

[spzala]

@devinrsmith that's true and this behavior is per design as it seems. The unsetting of the env variable ETCDCTL_API happens here:
https://github.com/etcd-io/etcd/blob/master/etcdctl/main.go#L33
Also, the usage of prefix ETCDCTL_ throws this error as an example:
2019-08-26 22:03:41.908806 W | pkg/flags: unrecognized environment variable ETCDCTL_X=3
https://github.com/etcd-io/etcd/blob/master//pkg/flags/flag.go#L86

I think documenting this behavior seems probably a good option. I will create a PR for it. Thanks!

/cc @xiang90 @gyuho