Investigate what it would take to allow users to create any arbitrary secondary index over `$queryStats`, depending on the given query filter.

For example, given:

[
  { $queryStats: {} },
  { $match: {
      "key.client.application.name": "checkout-api",
      "key.queryShape.cmdNs.db": "shop",
      "key.queryShape.cmdNs.coll": "orders"
  }}
]

the secondary index would rely on a compound index consisting of (key.client.application.name,
 key.queryShape.cmdNs.db,
 key.queryShape.cmdNs.coll) or something to that effect.

cc: jinfeng.ni@mongodb.com