There is a flag on the read path commands called includeQueryStatsMetrics that is set from mongos, that tells the mongods to append metrics so that mongos and collect and record query stats. We typically only expect this flag to be set internally.

The corresponding flag on the write path for updates is includeQueryStatsMetricsForOpIndex.

On the read path, we might let it through, or it might return an error, I'm not sure which. We should look at what the read path does regarding this flag and do the same thing. It is nice to be able to set this flag for testing the mongods directly, if it's possible to continue to allow that, it would be a nice-to-have.