tailcfg, logtail: provide Debug bit to disable logtail

For people running self-hosted control planes who want a global
opt-out knob instead of running their own logcatcher.

Change-Id: I7f996c09f45850ff77b58bfd5a535e197971725a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

logtail/logtail.go

@@ -20,6 +20,7 @@ import (
 
 	"tailscale.com/logtail/backoff"
 	"tailscale.com/net/interfaces"
+	"tailscale.com/syncs"
 	tslogger "tailscale.com/types/logger"
 	"tailscale.com/wgengine/monitor"
 )
@@ -412,7 +413,18 @@ func (l *Logger) Flush() error {
 	return nil
 }
 
+// logtailDisabled is whether logtail uploads to logcatcher are disabled.
+var logtailDisabled syncs.AtomicBool
+
+// Disable disables logtail uploads for the lifetime of the process.
+func Disable() {
+	logtailDisabled.Set(true)
+}
+
 func (l *Logger) send(jsonBlob []byte) (int, error) {
+	if logtailDisabled.Get() {
+		return len(jsonBlob), nil
+	}
 	n, err := l.buffer.Write(jsonBlob)
 	if l.drainLogs == nil {
 		select {