feat: flag to validate config files

[llamafilm]

Use Case

My telegraf instance receives new config files automatically from an external service, so I'd like to validate the config file works before running it. It should catch these errors:

• Invalid TOML
• Plugins which are not compiled into the binary
• Incorrect plugin options

This could be implemented as a new command-line flag --check-config. If it passes, it returns 0, and if it fails, it returns 78 (or any other code except 1) to differentiate this from a failed --test.

The --test flag is inadequate for this purpose for 3 reasons:

• It returns 1 if any of the inputs fail, even though the config is valid
• It takes a long time because it has to wait for each input to timeout
• It doesn't work in a dev environment that doesn't have network access to all the targets

Expected behavior

..

Actual behavior

..

Additional info

No response

[srebhan]

@llamafilm please check the binary in PR #15732 available once CI finished the tests. Please run telegraf with the config check subcommand and let me know if this fixes the issue!

[llamafilm]

Thanks! This works as expected, and takes care of all the issues I was thinking of. It returns 1 on failure, which is actually fine since this command would never be combined with --test.