Need the way to create custom metrics

Hi,

I need to know the way to implement custom metrics in my flink program.

Currently, I know we can create custom metrics with the help of RuntimeContext. 

But in my aggregate() I do not have RuntimeContext. I am using window operator and applying aggregate() method on it. And I am passing AggregateFunction in aggregate() method.

So, Kindly guide me, how can I create custom metrics in my code?

Note: As we know we can not user RichAggregateFunction with aggregate() method