Error: Data structure returned is not convertible in the requested format: Only integer keys supported for array columns on base level. Unsupported string 'metadata' found for row 'array (
  'metrics' => 
  array (
  ),
  'dimension' => 'Country',
  'columns' => 
  array (
    'nb_visits' => 'Visits',
    'nb_uniq_visitors' => 'Unique visitors',
    'nb_actions' => 'Actions',
    'nb_actions_per_visit' => 'Actions per Visit',
    'avg_time_on_site' => 'Avg. Time on Website',
    'bounce_rate' => 'Bounce Rate',
    'conversion_rate' => 'Conversion Rate',
  ),
)'. Try to call this method with the parameters '&format=original&serialize=1'; you will get the original php data structure serialized.