net/portmapper: fix invalid UPnP metric name

Fixes #9551

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I06f3a15a668be621675be6cbc7e5bdcc006e8570

net/portmapper/portmapper_test.go

@@ -124,3 +124,14 @@ func TestPCPIntegration(t *testing.T) {
 		t.Errorf("got nil mapping after successful createOrGetMapping")
 	}
 }
+
+// Test to ensure that metric names generated by this function do not contain
+// invalid characters.
+//
+// See https://github.com/tailscale/tailscale/issues/9551
+func TestGetUPnPErrorsMetric(t *testing.T) {
+	// This will panic if the metric name is invalid.
+	getUPnPErrorsMetric(100)
+	getUPnPErrorsMetric(0)
+	getUPnPErrorsMetric(-100)
+}