Would like server stats in mongos on scatter-gather queries

While monitoring a sharded cluster it would be helpful to know how seriously we are being affected by scatter-gather queries - how many operations are being targeted to multiple shards, how many require sorting and merging results on a shardsvr, and so forth.

While some of this data can be gathered by analyzing queries as they go in, it would be nice to be able to monitor a simple counter or two on grafana, CloudWatch, or Ops Manager to tell if some new code release or some other event has triggered a spate of scatter-gather queries that are negatively affecting performance.