sockstats: switch label to enum

Makes it cheaper/simpler to persist values, and encourages reuse of
labels as opposed to generating an arbitrary number.

Updates tailscale/corp#9230
Updates #3363

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

net/sockstats/label_string.go

@@ -0,0 +1,32 @@
+// Code generated by "stringer -type Label -trimprefix Label"; DO NOT EDIT.
+
+package sockstats
+
+import "strconv"
+
+func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[LabelControlClientAuto-0]
+	_ = x[LabelControlClientDialer-1]
+	_ = x[LabelDERPHTTPClient-2]
+	_ = x[LabelLogtailLogger-3]
+	_ = x[LabelDNSForwarderDoH-4]
+	_ = x[LabelDNSForwarderUDP-5]
+	_ = x[LabelNetcheckClient-6]
+	_ = x[LabelPortmapperClient-7]
+	_ = x[LabelMagicsockConnUDP4-8]
+	_ = x[LabelMagicsockConnUDP6-9]
+}
+
+const _Label_name = "ControlClientAutoControlClientDialerDERPHTTPClientLogtailLoggerDNSForwarderDoHDNSForwarderUDPNetcheckClientPortmapperClientMagicsockConnUDP4MagicsockConnUDP6"
+
+var _Label_index = [...]uint8{0, 17, 36, 50, 63, 78, 93, 107, 123, 140, 157}
+
+func (i Label) String() string {
+	if i >= Label(len(_Label_index)-1) {
+		return "Label(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _Label_name[_Label_index[i]:_Label_index[i+1]]
+}