add /api/tunnels/:id/status endpoint

Provides connected_sockets information for a tunnel.

lib/TunnelAgent.test.js

@@ -173,4 +173,11 @@ describe('TunnelAgent', () => {
         });
         assert.equal(err.message, 'foo');
     });
+
+    it('should return stats', async () => {
+        const agent = new TunnelAgent();
+        assert.deepEqual(agent.stats(), {
+            connectedSockets: 0,
+        });
+    });
 });