Add structured logging support to promhttp

[SuperQ]

In order to better support the standard library log/slog add a new interface to the promhttp HandlerOpts.

[tjhop]

\o/

[SuperQ]

We might want to split out the Go version change into a separate PR.

[SuperQ]

Ok, yaks shaven, this is ready to go.

[SuperQ]

Seems like dagger is a bit of technical debt for keeping Go versions up to date as well.

[ArthurSens]

Hmmm, feels strange that we have 2 loggers in the struct now. I'm assuming this is needed somewhere in a different package/repository, could you clarify how you're planning to use this?

[tjhop]

Hmmm, feels strange that we have 2 loggers in the struct now. I'm assuming this is needed somewhere in a different package/repository, could you clarify how you're planning to use this?

@SuperQ we're only ever logging at error level here -- maybe we just lean on slog's built in stdlog adapter and avoid the change here altogether? Something like this:

	// make logger with promslog like normal
	slogger := promslog.New(&promslog.Config{})
	// make stdlog adapter from slogger
	logger := slog.NewLogLogger(slogger.Handler(), slog.LevelError)

Bonus points -- log output is still structured

[SuperQ]

Yea, I suppose using a stdlib adapter would be fine.