feat: Support OTel and JSON for logs (via log/slog) (#760)

Co-authored-by: Kyle Mendell <kmendell@ofkm.us>

backend/internal/utils/slogfanout.go

@@ -0,0 +1,85 @@
+package utils
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"log/slog"
+	"slices"
+)
+
+// This file contains code adapted from https://github.com/samber/slog-multi
+// Source: https://github.com/samber/slog-multi/blob/ced84707f45ec9848138349ed58de178eedaa6f2/pipe.go
+// Copyright (C) 2023 Samuel Berthe
+// License: MIT (https://github.com/samber/slog-multi/blob/ced84707f45ec9848138349ed58de178eedaa6f2/LICENSE)
+
+// LogFanoutHandler is a slog.Handler that sends logs to multiple destinations
+type LogFanoutHandler []slog.Handler
+
+// Implements slog.Handler
+func (h LogFanoutHandler) Enabled(ctx context.Context, l slog.Level) bool {
+	for i := range h {
+		if h[i].Enabled(ctx, l) {
+			return true
+		}
+	}
+
+	return false
+}
+
+// Implements slog.Handler
+func (h LogFanoutHandler) Handle(ctx context.Context, r slog.Record) error {
+	errs := make([]error, 0)
+	for i := range h {
+		if h[i].Enabled(ctx, r.Level) {
+			err := try(func() error {
+				return h[i].Handle(ctx, r.Clone())
+			})
+			if err != nil {
+				errs = append(errs, err)
+			}
+		}
+	}
+
+	return errors.Join(errs...)
+}
+
+// Implements slog.Handler
+func (h LogFanoutHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
+	res := make(LogFanoutHandler, len(h))
+	for i, v := range h {
+		res[i] = v.WithAttrs(slices.Clone(attrs))
+	}
+	return res
+}
+
+// Implements slog.Handler
+func (h LogFanoutHandler) WithGroup(name string) slog.Handler {
+	// https://cs.opensource.google/go/x/exp/+/46b07846:slog/handler.go;l=247
+	if name == "" {
+		return h
+	}
+
+	res := make(LogFanoutHandler, len(h))
+	for i, v := range h {
+		res[i] = v.WithGroup(name)
+	}
+	return res
+}
+
+func try(callback func() error) (err error) {
+	defer func() {
+		r := recover()
+		if r != nil {
+			if e, ok := r.(error); ok {
+				err = e
+			} else {
+				err = fmt.Errorf("unexpected error: %+v", r)
+			}
+		}
+	}()
+
+	err = callback()
+
+	return
+}