Each query that runs through the sampling estimator currently holds a sample in memory. Currently, the memory usage of that sample is already computed in SamplingEstimatorImpl::getSamplingMetadata() but this is only exposed in explain:

• It is not charged to the operation's memory tracker, so it does not show up in peakTrackedMemBytes.
• No serverStatus metric reports the memory size of the sample bytes so this information is not available in FTDC.

This would be an extremely valuable tool in diagnosing future HELP tickets that may arise.