Challenge 2: Add a $match Filter — Possible Solution
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db.products.aggregate([
  { $match: { in_stock: true } },
  { $group: {
      _id: "$category",
      product_count: { $sum: 1 },
      avg_price: { $avg: "$price" }
  } },
  { $sort: { product_count: -1 } }
])

WHY THIS WORKS AS AN ANSWER
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The new $match stage goes FIRST, before $group — filtering out-of-stock
products before they're ever counted or averaged, the same as SQL
evaluating WHERE before GROUP BY.

Position matters here for two reasons, not just one:

1. Correctness: if $match ran AFTER $group, there would be no
   in_stock field left to filter on — $group's output documents only
   contain _id, product_count, and avg_price, since $group discards
   every field it wasn't told to keep or compute. The filter would
   either error or silently match nothing.

2. Performance: putting $match first lets MongoDB filter out documents
   (and use an index on in_stock, if one exists) before doing any
   grouping work at all, rather than grouping the entire collection
   and only then throwing rows away.