logging: Implement append encoder, allow flatter filters config

[francislavoie]

Feature requested by @yroc92

This adds a simple encoder called append which can append one or more log fields to all log entries that the logger processes.

This is inspired (and largely copy-paste) of the filter encoder, and as such it wraps another encoder (defaulting to json or console as per #5980).

Since this module does not have access to the request context, it cannot use request placeholders, but it may use global placeholders such as the ones from https://caddyserver.com/docs/conventions#placeholders.

This is complementary to #6066 which allows appending extra log fields to access logs only, and allows appending data from the request context.

Most commonly, this will probably be used to append things like a Caddy instance ID, to identify which instance the log was produced from in some log ingress tool.

For example, in global options:

{
	log {
		format append {
			fields {
				machine {env.MACHINE}
			}
		}
	}
}

And running with MACHINE=foo caddy run, a log like this is shown:

2024/01/28 06:39:51.145	INFO	serving initial configuration	{"machine": "foo"}

The key functionality of this encoder is in the func (fe AppendEncoder) EncodeEntry function (near the end of the file). The rest of the code is pretty much just zap boilerplate because we need to implement the zapcore.Encoder interface which has a ton of functions 😬 we're just passing them through to the wrapped encoder.

[francislavoie]

I just added another commit which makes it possible to configure fields for both filter and append encoders in a flatter way, allowing to skip the fields { } wrapper. The wrapper is only needed if you ever have a field you want to call wrap or fields 😅

Before:

{
	log {
		format append {
			fields {
				machine {env.MACHINE}
			}
		}
	}
}

After:

{
	log {
		format append {
			machine {env.MACHINE}
		}
	}
}

[mholt]

Oh -- lol, would it be prudent to rename this to AppendEncoder or would that be terribly annoying? 😅

[francislavoie]

Okay 👍

[mohammed90]

Francis,