Implement go_slog-style logging in go-loggly

- Ref: https://github.com/GigaOM/legacy-pro/issues/3806
- https://github.com/okredo/go-loggly/commit/71f22c94f3a49a06277b7a403f3a64053e56d2e0 is step #1 of [above](https://github.com/GigaOM/legacy-pro/issues/3806)
- https://github.com/okredo/go-loggly/commit/3cd0cfe22150cda32e461ca9243695761da42e04 is step #2 of [above](https://github.com/GigaOM/legacy-pro/issues/3806)
I'd call it `$backtrace` instead of `$functions`.