Format % values for plugin's metrics, cherry pick...

Format % values for plugin's metrics, cherry pick https://github.com/piwik/piwik/commit/d4d87dbfe90a4e6502dfc57ee6bfe61b67435b7c

plugins/API/ProcessedReport.php

@@ -790,10 +790,10 @@ class ProcessedReport
         // Add % symbol to rates
         if (strpos($columnName, '_rate') !== false) {
             if (strpos($value, "%") === false) {
-                return $value . "%";
+                return (100 * $value) . "%";
             }
         }
 
         return $value;
     }
-}
+}
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