From the design document, the following changes should be made to $indexStats:

• By default, each document returned by $indexStats will return an additional spec field that will contain the BSON object index description associated with the index in its IndexDescriptor, like those returned by the listIndexes command.
• Indexes that are being built should be included in the $indexStats output and will contain an additional building boolean field. This field will not be present for indexes that have finished building.
• Output documents generated for indexes on sharding enabled node started with --shardsvr should contain the shardId of the shard that generated the document in a new shard field.