etcdctl: add command to generate shell completion

[avorima]

To improve the UX of etcdctl. Completion is generated by cobra
according to defined commands and flags.

Fixes #13111

[ptabor]

Nice. Could you please add an tests/e2e/ctl_v3_*.go test to cover the command is working (even on individual example).

[lilic]

Thanks! Works like a charm 🎉 👍

Would you mind adding it for etcdutl in a follow up maybe?

@ptabor should we add this to the release notes as well what do you think?

[ptabor]

@ptabor +1 for release notes. The notable first entry for 3.6.

[avorima]

Nice. Could you please add an tests/e2e/ctl_v3_*.go test to cover the command is working (even on individual example).

I'll look into writing tests. Do you mean one for one shell only?

Would you mind adding it for etcdutl in a follow up maybe?

Sure, I can do that.

[ptabor]

Nice. Could you please add an tests/e2e/ctl_v3_*.go test to cover the command is working (even on individual example).

I'll look into writing tests. Do you mean one for one shell only?

Yes - mostly to make sure someone didn't turned off the group incidentally.

Would you mind adding it for etcdutl in a follow up maybe?

Sure, I can do that.

[avorima]

@ptabor I've added a really basic test that verifies the generated shell code is valid. Testing the completion itself is kind of tricky and probably not feasible in a non-interactive environment. I could add some test cases for the hidden __complete command that the shell code uses though.

The test is for bash, but can easily be extended for zsh and fish. They just have to be installed in CI.

[ptabor]

Great . Thank you.

[lilic]

Thank you! 🎉