title: SAPUI5 control metadata, adding custom attributes
body: <p>I am working on adding some common functionality to a potentially large set of diverse custom controls.</p>
  
  <p>To surface my configuration-driven functionality, I was thinking an elegant way could be to add custom members to the "metadata" section of a control's definition.</p>
  
  <p>For example, something like this:</p>
  
  <pre><code>sap.ui.core.Control.extend("myCompany.foo.FooControl", {
      metadata: {
          // normal stuff, properties, aggregation, etc...
          properties: {
          },
          ...
  
          // my custom stuff
          custom: {
          }
      }
  });
  </code></pre>
  
  <p>Is there a best practice way of adding custom content to the metadata object? 
  Or, is there a way to get a hold of this object literal at runtime so I could at least parse it for my custom attributes?</p>