log/slog: add GroupAttrs

GroupAttrs is a more efficient version of Group
that takes a slice of Attr values.

Fixes #66365

Change-Id: Ic3046704825e17098f2fea5751f2959dce1073e2
Reviewed-on: https://go-review.googlesource.com/c/go/+/672915
Reviewed-by: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

api/next/66365.txt

@@ -0,0 +1 @@
+pkg log/slog, func GroupAttrs(string, ...Attr) Attr #66365

doc/next/6-stdlib/99-minor/log/slog/66365.md

@@ -0,0 +1 @@
+[GroupAttrs] creates a group [Attr] from a slice of [Attr] values.

src/log/slog/attr.go

@@ -67,6 +67,15 @@ func Group(key string, args ...any) Attr {
 	return Attr{key, GroupValue(argsToAttrSlice(args)...)}
 }
 
+// GroupAttrs returns an Attr for a Group [Value]
+// consisting of the given Attrs.
+//
+// GroupAttrs is a more efficient version of [Group]
+// that accepts only [Attr] values.
+func GroupAttrs(key string, attrs ...Attr) Attr {
+	return Attr{key, GroupValue(attrs...)}
+}
+
 func argsToAttrSlice(args []any) []Attr {
 	var (
 		attr  Attr

src/log/slog/example_test.go

@@ -5,6 +5,7 @@
 package slog_test
 
 import (
+	"context"
 	"log/slog"
 	"net/http"
 	"os"
@@ -35,3 +36,46 @@ func ExampleGroup() {
 	// Output:
 	// level=INFO msg=finished req.method=GET req.url=localhost status=200 duration=1s
 }
+
+func ExampleGroupAttrs() {
+	r, _ := http.NewRequest("POST", "localhost", http.NoBody)
+	// ...
+
+	logger := slog.New(
+		slog.NewTextHandler(os.Stdout, &slog.HandlerOptions{
+			Level: slog.LevelDebug,
+			ReplaceAttr: func(groups []string, a slog.Attr) slog.Attr {
+				if a.Key == slog.TimeKey && len(groups) == 0 {
+					return slog.Attr{}
+				}
+				return a
+			},
+		}),
+	)
+
+	// Use []slog.Attr to accumulate attributes.
+	attrs := []slog.Attr{slog.String("method", r.Method)}
+	attrs = append(attrs, slog.String("url", r.URL.String()))
+
+	if r.Method == "POST" {
+		attrs = append(attrs, slog.Int("content-length", int(r.ContentLength)))
+	}
+
+	// Group the attributes under a key.
+	logger.LogAttrs(context.Background(), slog.LevelInfo,
+		"finished",
+		slog.Int("status", http.StatusOK),
+		slog.GroupAttrs("req", attrs...),
+	)
+
+	// Groups with empty keys are inlined.
+	logger.LogAttrs(context.Background(), slog.LevelInfo,
+		"finished",
+		slog.Int("status", http.StatusOK),
+		slog.GroupAttrs("", attrs...),
+	)
+
+	// Output:
+	// level=INFO msg=finished status=200 req.method=POST req.url=localhost req.content-length=0
+	// level=INFO msg=finished status=200 method=POST url=localhost content-length=0
+}