logtail: add Logger.SetEnabled to toggle uploads at runtime

Callers that need to turn logtail uploads on and off in response to
user preference or policy changes previously had no choice: the
package-level Disable is a one-way kill switch intended for the
controlplane DisableLogTail debug message, and requires a process
restart to undo.

Add a per-Logger disabled flag, toggled via SetEnabled, that drops
incoming entries without buffering while disabled. The process-wide
Disable still takes precedence, so a controlplane-issued kill switch
cannot be overridden by a client setting it back on.

To simplify https://github.com/tailscale/tailscale-android/pull/695

Updates #13174

Change-Id: I06e75bd719c851f5f837ca5b2d1e17f7c68355f0
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

logtail/logtail_test.go

@@ -321,6 +321,37 @@ func TestLoggerWriteResult(t *testing.T) {
 	}
 }
 
+func TestLoggerSetEnabled(t *testing.T) {
+	buf := NewMemoryBuffer(100)
+	lg := &Logger{
+		clock:  tstest.NewClock(tstest.ClockOpts{Start: time.Unix(123, 0)}),
+		buffer: buf,
+	}
+
+	if _, err := lg.Write([]byte("enabled1")); err != nil {
+		t.Fatal(err)
+	}
+	if back, _ := buf.TryReadLine(); !strings.Contains(string(back), "enabled1") {
+		t.Fatalf("initial write not buffered; got %q", back)
+	}
+
+	lg.SetEnabled(false)
+	if _, err := lg.Write([]byte("disabled")); err != nil {
+		t.Fatal(err)
+	}
+	if back, _ := buf.TryReadLine(); len(back) != 0 {
+		t.Errorf("write while disabled leaked into buffer: %q", back)
+	}
+
+	lg.SetEnabled(true)
+	if _, err := lg.Write([]byte("enabled2")); err != nil {
+		t.Fatal(err)
+	}
+	if back, _ := buf.TryReadLine(); !strings.Contains(string(back), "enabled2") {
+		t.Errorf("write after re-enable not buffered; got %q", back)
+	}
+}
+
 func TestAppendMetadata(t *testing.T) {
 	var lg Logger
 	lg.clock = tstest.NewClock(tstest.ClockOpts{Start: time.Date(2000, 01, 01, 0, 0, 0, 0, time.UTC)})