Engine log level propagation

[denis256]

Description

Included changes:

• added propagation of log level to the engine through env variables
• added env var documentation
• added tests to check log passing
• updated handling of env vars through flags
• simplified tests

#3103

TODOs

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• Update the docs.
• Run the relevant tests successfully, including pre-commit checks.
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• Include release notes. If this PR is backward incompatible, include a migration guide.

Release Notes (draft)

Added / Removed / Updated [X].

Migration Guide

[yhakbar]

@denis256 , Please remove fatal and panic here. We removed those as valid log levels in Terragrunt, so it's a good idea to remove them from the docs here too. We'll probably eventually support stdout and stderr as valid log levels.

[denis256]

Removed panic and fatal.

[yhakbar]

Consider whether you need special behavior to handle this:

terragrunt/cli/commands/flags.go

Lines 352 to 361 in 58e2c20

	&cli.BoolFlag{
	Name: TerragruntLogDisableFlagName,
	EnvVar: TerragruntLogDisableEnvName,
	Usage: "Disable logging",
	Action: func(ctx *cli.Context, _ bool) error {
	opts.ForwardTFStdout = true
	opts.Logger.SetOptions(log.WithFormatter(&format.SilentFormatter{}))
	return nil
	},
	},

We might want to suppress logging in the engine too when it's disabled in Terragrunt.

[denis256]

Added setting of "off" log level

[yhakbar]

This is different. Disabling log formatting only makes the logs output in key-value pairs, it doesn't stop them from logging altogether.

[levkohimins]

It will not work with, false, 0, and will not notify about a wrong value. The same with TG_ENGINE_CACHE_PATH, TG_ENGINE_SKIP_CHECK.

In this particular case I would recommend adding opts.DisableLog

		&cli.BoolFlag{
			Name:   TerragruntLogDisableFlagName,
			EnvVar: TerragruntLogDisableEnvName,
			Destination: &opts.DisableLog,

Let's agree that the only entry point for all environment variables is CLI flags, this will help avoid issues in the future, when we forget about the existence of this "orphan", and it will live its own life. For example, when globally renaming a prefix or redesign the CLI interface. If for some reason you don't need to show the evn variable flag in the CLI help, you can simply hide it from users with Hidden: true, but they all will be in one place, and we will know about them.

[levkohimins]

Great job!