SET SESSION_STATES will not change the resource group connection count

[YangKeao]

Bug Report

1. Minimal reproduce step (Required)

The following test will fail:

	// The resource group set by `SET SESSION_STATE` will be tracked by the counter
	// At first, create a new cert/key pair to encode session state
	tempDir := t.TempDir()
	certPath := filepath.Join(tempDir, "cert.pem")
	keyPath := filepath.Join(tempDir, "key.pem")
	err := util.CreateCertificates(certPath, keyPath, 1024, x509.RSA, x509.UnknownSignatureAlgorithm)
	require.NoError(t, err)

	sessionstates.SetCertPath(certPath)
	sessionstates.SetKeyPath(keyPath)
	sessionstates.ReloadSigningCert()
	cli.RunTests(t, nil, func(dbt *testkit.DBTestKit) {
		ctx := context.Background()
		conn, err := dbt.GetDB().Conn(ctx)
		require.NoError(t, err)
		resourceGroupConnCountReached(t, "default", 1.0)
		// Now set the resource group to `test`
		_, err = conn.ExecContext(ctx, "set resource group test")
		require.NoError(t, err)
		resourceGroupConnCountReached(t, "default", 0.0)
		resourceGroupConnCountReached(t, "test", 1.0)

		// Encode the session state
		rows, err := conn.QueryContext(ctx, "show session_states")
		require.NoError(t, err)
		var sessionStates, signInfo string
		rows.Next()
		err = rows.Scan(&sessionStates, &signInfo)
		require.NoError(t, err)
		require.NoError(t, rows.Close())

		// Now reset the resource group to `default`
		_, err = conn.ExecContext(ctx, "set resource group default")
		require.NoError(t, err)
		resourceGroupConnCountReached(t, "default", 1.0)
		resourceGroupConnCountReached(t, "test", 0.0)
		// Set the session state
		sessionStates = strings.ReplaceAll(sessionStates, "\\", "\\\\")
		sessionStates = strings.ReplaceAll(sessionStates, "'", "\\'")
		_, err = conn.ExecContext(ctx, fmt.Sprintf("set session_states '%s'", sessionStates))
		require.NoError(t, err)
		resourceGroupConnCountReached(t, "default", 0.0)
		resourceGroupConnCountReached(t, "test", 1.0)
	})

Instead, the count of default resource group is always 1.