Description

We use Druid's Groupby queries heavily. These Groupby queries use the merge buffers to merge results from different historicals, real-time peons/indexers. We currently have the metric for the number of merge buffers, but we don't know whether we're optimizing the size of these merge buffers. Currently, we're setting it to 512 Mb, but we suspect that we can reduce it. We can't take such an action without having concrete metrics.
It would be nice if we have a metric indicating the bytes used for merge buffers.

Motivation

The above description already has the motivation behind this feature.