tailcfg: optimize keyMarshalText

This function accounted for ~1% of all allocs by tailscaled.
It is trivial to improve, so may as well.

name              old time/op    new time/op    delta
KeyMarshalText-8     197ns ± 0%      47ns ± 0%  -76.12%  (p=0.016 n=4+5)

name              old alloc/op   new alloc/op   delta
KeyMarshalText-8      200B ± 0%       80B ± 0%  -60.00%  (p=0.008 n=5+5)

name              old allocs/op  new allocs/op  delta
KeyMarshalText-8      5.00 ± 0%      1.00 ± 0%  -80.00%  (p=0.008 n=5+5)

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

tailcfg/tailcfg.go

@@ -7,7 +7,7 @@
 //go:generate go run tailscale.com/cmd/cloner --type=User,Node,Hostinfo,NetInfo,Login,DNSConfig,DNSResolver,RegisterResponse --clonefunc=true --output=tailcfg_clone.go
 
 import (
-	"bytes"
+	"encoding/hex"
 	"errors"
 	"fmt"
 	"reflect"
@@ -1027,9 +1027,10 @@ func (k MachineKey) HexString() string                { return fmt.Sprintf("%x",
 func (k *MachineKey) UnmarshalText(text []byte) error { return keyUnmarshalText(k[:], "mkey:", text) }
 
 func keyMarshalText(prefix string, k [32]byte) []byte {
-	buf := bytes.NewBuffer(make([]byte, 0, len(prefix)+64))
-	fmt.Fprintf(buf, "%s%x", prefix, k[:])
-	return buf.Bytes()
+	buf := make([]byte, len(prefix)+64)
+	copy(buf, prefix)
+	hex.Encode(buf[len(prefix):], k[:])
+	return buf
 }
 
 func keyUnmarshalText(dst []byte, prefix string, text []byte) error {

tailcfg/tailcfg_test.go

@@ -518,3 +518,13 @@ func TestEndpointTypeMarshal(t *testing.T) {
 		t.Errorf("got %s; want %s", got, want)
 	}
 }
+
+var sinkBytes []byte
+
+func BenchmarkKeyMarshalText(b *testing.B) {
+	b.ReportAllocs()
+	var k [32]byte
+	for i := 0; i < b.N; i++ {
+		sinkBytes = keyMarshalText("prefix", k)
+	}
+}