logtail: increase maximum log line size in low memory mode

The 255 byte limit was chosen more than 3 years ago (tailscale/corp@929635c9d9),
when iOS was operating under much more significant memory constraints.
With iOS 15 the network extension has an increased limit, so increasing
it to 4K should be fine.

The motivating factor was that the network interfaces being logged
by linkChange in wgengine/userspace.go were getting truncated, and it
would be useful to know why in some cases we're choosing the pdp_ip1
cell interface instead of the pdp_ip0 one.

Updates #7184
Updates #7188

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

logtail/logtail.go

@@ -558,7 +558,7 @@ func (l *Logger) encodeText(buf []byte, skipClientTime bool, procID uint32, proc
 	// Put a sanity cap on buf's size.
 	max := 16 << 10
 	if l.lowMem {
-		max = 255
+		max = 4 << 10
 	}
 	var nTruncated int
 	if len(buf) > max {

logtail/logtail_test.go

@@ -189,7 +189,7 @@ func TestEncodeSpecialCases(t *testing.T) {
 	// lowMem + long string
 	l.skipClientTime = false
 	l.lowMem = true
-	longStr := strings.Repeat("0", 512)
+	longStr := strings.Repeat("0", 5120)
 	io.WriteString(l, longStr)
 	body = <-ts.uploaded
 	data = unmarshalOne(t, body)
@@ -197,8 +197,8 @@ func TestEncodeSpecialCases(t *testing.T) {
 	if !ok {
 		t.Errorf("lowMem: no text %v", data)
 	}
-	if n := len(text.(string)); n > 300 {
-		t.Errorf("lowMem: got %d chars; want <300 chars", n)
+	if n := len(text.(string)); n > 4500 {
+		t.Errorf("lowMem: got %d chars; want <4500 chars", n)
 	}
 
 	// -------------------------------------------------------------------------
@@ -332,10 +332,10 @@ func unmarshalOne(t *testing.T, body []byte) map[string]any {
 
 func TestEncodeTextTruncation(t *testing.T) {
 	lg := &Logger{timeNow: time.Now, lowMem: true}
-	in := bytes.Repeat([]byte("a"), 300)
+	in := bytes.Repeat([]byte("a"), 5120)
 	b := lg.encodeText(in, true, 0, 0, 0)
 	got := string(b)
-	want := `{"text": "` + strings.Repeat("a", 255) + `…+45"}` + "\n"
+	want := `{"text": "` + strings.Repeat("a", 4096) + `…+1024"}` + "\n"
 	if got != want {
 		t.Errorf("got:\n%qwant:\n%q\n", got, want)
 	}