Add Logger.StdLogger() to get a standard log.Logger

This is meant primarily for passing to APIs that only accept a
log.Logger as a custom logger.

alog_test.go

@@ -104,3 +104,22 @@ func TestIgnoredTag(t *testing.T) {
 		t.Fatalf("want: %#q, got: %#q", want, got)
 	}
 }
+
+func TestStdLogger(t *testing.T) {
+	buf := &bytes.Buffer{}
+	dumper := EmitterFunc(func(ctx context.Context, e *Entry) {
+		fmt.Fprintf(buf, "%s %v %s\n", e.Time.Format(time.RFC3339), e.Tags, e.Msg)
+	})
+
+	l := New(WithEmitter(dumper), OverrideTimestamp(func() time.Time { return time.Time{} }))
+
+	ctx := AddTags(context.Background(), "a", "b")
+	sl := l.StdLogger(ctx)
+	sl.Printf("test")
+
+	want := "0001-01-01T00:00:00Z [[a b]] test\n"
+	got := buf.String()
+	if got != want {
+		t.Fatalf("want: %#q, got: %#q", want, got)
+	}
+}

logger.go

@@ -3,6 +3,7 @@ package alog
 import (
 	"context"
 	"fmt"
+	"log"
 	"runtime"
 	"time"
 )
@@ -94,3 +95,21 @@ func (l *Logger) Print(ctx context.Context, v ...interface{}) {
 func (l *Logger) Printf(ctx context.Context, f string, v ...interface{}) {
 	l.Output(ctx, 2, fmt.Sprintf(f, v...))
 }
+
+type writerFunc func(p []byte) (int, error)
+
+func (w writerFunc) Write(p []byte) (int, error) {
+	return w(p)
+}
+
+// StdLogger returns a standard log.Logger that sends log messages to this
+// logger with the provided Context. The returned log.Logger should not be
+// modified.
+func (l *Logger) StdLogger(ctx context.Context) *log.Logger {
+	return log.New(writerFunc(func(p []byte) (int, error) {
+		// standard Logger always writes a trailing \n, so remove it
+		p = p[:len(p)-1]
+		l.Output(ctx, 4, string(p))
+		return len(p), nil
+	}), "", 0)
+}