In  SERVER-115694  and SERVER-117169 we added these counters to serverStatus:

• to the existing 'metrics.commands.aggregate' we will add this section:

withExtension: {
    succeeded: NumberLong(0),
    failed: NumberLong(0),
}

It is a bit tricky to implement so I wanted to pause and consider - should usage of extensions inside views count towards these? I think it probably should, but warrants an extra ticket anyway, as it looks like we'll have to plumb through some context, and potentially add some 'withExtension' sections to find and other commands that can work on views.