AdvancedDataGrid.dataProvider is an Object, so there is not length property. You have to cast it to something else.
The length property should work. Are you getting an error message?
The length property will work only if the underlying data provider object actually has it.
After playing around for a bit, I found that it does work. It just is not listed in the code hinting as an option anymore. I created a <s:callresponder result event and put it in there.
<s:CallResponder id="getAllResult" result="getAllResult_resultHandler(event)"/>
protected function getAllResult_resultHandler(event:ResultEvent):void
this.lblCount.text = this.adg1.dataProvider.length;