Feature Enhancement: publish keyspace as part of plugin help

[jspaleta]

Make it possible for the sdk to communicate the keyspace in the help message.

Straw proposal:

override the cobra.command default HelpFunction with a custom function that appends a Keyspace usage string into the the help output.

[nikkictl]

@jspaleta can you provide an example use case for this suggestion?

[jspaleta]

Straw use case example (slack handler):

Sensu slack handler currently defines a keyspace but its only documented in the readme..

It would be very useful if there was a note in the usage message for all plugins that have a Keyspace defined, indicated what the keyspace is, and if possible which arguments have a keyspace PATH defined. Not all arguments are keyspaced. Path strings associated with PluginConfigOption aren't discoverable from the Argument name.

Updated usage for the slack handler would look something like this:

The Sensu Go Slack handler for notifying a channel

Usage:
  sensu-slack-handler [flags]
  sensu-slack-handler [command]

Available Commands:
  help        Help about any command
  version     Print the version number of this plugin

Flags:
  -c, --channel string                The channel to post messages to (default "#general")
  -t, --description-template string   The Slack notification output template, in Golang text/template format (default "{{ .Check.Output }}")
  -h, --help                          help for sensu-slack-handler
  -i, --icon-url string               A URL to an image to use as the user avatar (default "https://www.sensu.io/img/sensu-logo.png")
  -u, --username string               The username that messages will be sent as (default "sensu")
  -w, --webhook-url string            The webhook url to send messages to

Annotation Keyspace:  "sensu.io/plugins/slack/config/<parameter>"
  Keyspace active for the following parameters:
    webHookURL,
    channel,
    username,
    iconURL,
    descriptionTemplate

Use "sensu-slack-handler [command] --help" for more information about a command.
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