enhancement(cli): Shell autocomplete for the Vector CLI

[lucperkins]

This is currently a draft, as I found out after submitting that attempting to generate zsh completions throws a fatal panic due to an issue in clap. This has been fixed in clap (see this PR). Those changes have not, however, made it into a non-beta release and thus haven't been yet incorporated into structopt. Once structopt has incorporated those changes, we should be able to push this through without issue; until then, a panic on vector completion zsh is likely to surprise and frustrate users, so let's keep this on ice for the time being.

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[hhromic]

I was just using the VRL shell to file a bug report and was getting so mad of not having TAB-autocompletion, specially when auto-completion is already possible by using the RightArrow key instead of TAB. Thanks so much for this!

EDIT: Wait, the RightArrow key is not actually autocompletion... so thanks even more for this!

[lucperkins]

@hhromic Sadly, I pushed this having only tested Bash and Fish, which worked great, but now I see that Zsh panics with a completely inscrutable error. Sigh. I'll see what I can do, but if this requires an overhaul of our CLI plumbing this may not come through for a while.

[hhromic]

@lucperkins apologies, I just realised I completely misunderstood this PR 🤦

You are implementing autocompletion for the vector command itself for the user shell!
I thought you were implemeting autocompletion for the VRL shell itself, i.e. inside vector vrl.

For example this sort of thing:

$ parse_ [tab]
parse_apache_log  parse_key_value  parse_syslog ...

$ parse_key_value(message, key [tab]
key_value_delimiter

$ parse_key_value(message, key_value_delimiter: ",", accept_ [tab]
accept_standalone_key

(that last one I never remember the exact name so I often have to go look it up in the docs!)

[lucperkins]

@hhromic Ah yes, this is just for the CLI. Inside the VRL REPL there is only limited completion. I may take a stab at that at some point but I can't say it's high priority at the moment. In the meantime, if you're writing VRL in separate .vrl files there is a VS Code extension that offers tab completion for function names.

I should also note that in the VRL REPL there are some helper commands that you can see if you run help. help functions (or help funcs or help fs) displays a list of all available functions. To open up the docs for a specific function in your browser you can run help docs <function>, e.g. help docs parse_json. I know that doesn't match 1:1 with what you're seeking but I've personally found it pretty useful to remember names like parse_aws_cloudwatch_log_subscription_message.

[lucperkins]

Closing this for now, as it's unclear when the necessary dependency updates will happen. But I'll keep checking back on that and reopen this when the time comes.