docs: add 10 missing aggregation pipeline stage references

api-reference/operators/aggregation/$count.md

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+---
+title: $count
+description: The $count stage returns a count of the number of documents at this stage of the aggregation pipeline.
+type: operators
+category: aggregation
+---
+
+# $count
+
+The `$count` stage passes a document to the next stage that contains a count of the number of documents input to the stage. This is useful for getting the total number of documents that match earlier pipeline stages.
+
+## Syntax
+
+```javascript
+{
+  $count: <string>
+}
+```
+
+## Parameters
+
+| Parameter | Description |
+| --- | --- |
+| **`string`** | Required. The name of the output field which has the count as its value. Must be a non-empty string, must not start with `$`, and must not contain the `.` character. |
+
+## Examples
+
+Consider this sample document from the stores collection.
+
+```json
+{
+  "_id": "2cf3f885-9962-4b67-a172-aa9039e9ae2f",
+  "name": "First Up Consultants | Bed and Bath Center - South Amir",
+  "location": {
+    "lat": 60.7954,
+    "lon": -142.0012
+  },
+  "staff": {
+    "totalStaff": {
+      "fullTime": 18,
+      "partTime": 17
+    }
+  },
+  "sales": {
+    "totalSales": 37701
+  }
+}
+```
+
+### Example 1: Count all documents
+
+Count the total number of documents in the collection:
+
+```javascript
+db.stores.aggregate([
+  { $count: "totalStores" }
+])
+```
+
+This query returns:
+
+```json
+[
+  { "totalStores": 5 }
+]
+```
+
+### Example 2: Count documents after filtering
+
+Count stores that have more than 10 full-time staff:
+
+```javascript
+db.stores.aggregate([
+  { $match: { "staff.totalStaff.fullTime": { $gt: 10 } } },
+  { $count: "highStaffStores" }
+])
+```
+
+This query returns:
+
+```json
+[
+  { "highStaffStores": 3 }
+]
+```
+
+### Example 3: Count with $unwind
+
+Count the total number of promotion events across all stores:
+
+```javascript
+db.stores.aggregate([
+  { $unwind: "$promotionEvents" },
+  { $count: "totalPromotionEvents" }
+])
+```
+
+## Key Takeaways
+
+- **Single output document** — `$count` always returns exactly one document with a single field
+- **Close to, but not the same as, `$group`** — `{ $count: "total" }` resembles `{ $group: { _id: null, total: { $sum: 1 } } }` followed by `{ $project: { _id: 0 } }`, and the two agree whenever any document reaches the stage. They diverge on empty input: `$count` builds an ungrouped aggregate, which always produces a row, so it returns `{ "total": 0 }`, while `$group` performs a real grouping and returns nothing at all. Use `$count` when a downstream stage depends on always receiving a document.
+- **Field name restrictions** — the output field name must be non-empty, cannot start with `$`, and cannot contain `.`

api-reference/operators/aggregation/$currentop.md

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+---
+title: $currentOp
+description: The $currentOp stage returns information on active and queued operations for the database.
+type: operators
+category: aggregation
+---
+
+# $currentOp
+
+The `$currentOp` stage returns a stream of documents containing information on active and queued operations for the database instance. This stage must be the first stage in the pipeline and is run on the `admin` database.
+
+## Syntax
+
+```javascript
+db.adminCommand({
+  aggregate: 1,
+  pipeline: [
+    {
+      $currentOp: {
+        allUsers: <boolean>,
+        idleConnections: <boolean>,
+        idleCursors: <boolean>,
+        idleSessions: <boolean>,
+        localOps: <boolean>
+      }
+    }
+  ],
+  cursor: {}
+})
+```
+
+## Parameters
+
+| Parameter | Description |
+| --- | --- |
+| **`allUsers`** | Optional. Boolean. If `true`, reports operations for all users. Default: `false`. |
+| **`idleConnections`** | Optional. Boolean. If `true`, reports on idle connections. Default: `false`. |
+| **`idleCursors`** | Optional. Boolean. If `true`, reports on idle cursors. Default: `false`. |
+| **`idleSessions`** | Optional. Boolean. If `true`, reports on idle sessions. Default: `false`. |
+| **`localOps`** | Optional. Boolean. If `true`, reports operations running locally on the current instance. Default: `false`. |
+
+## Examples
+
+### Example 1: List active operations
+
+Return all currently active operations:
+
+```javascript
+db.adminCommand({
+  aggregate: 1,
+  pipeline: [
+    { $currentOp: { allUsers: true } }
+  ],
+  cursor: {}
+})
+```
+
+### Example 2: Filter active operations
+
+Return active operations for a specific database, combined with `$match`:
+
+```javascript
+db.adminCommand({
+  aggregate: 1,
+  pipeline: [
+    { $currentOp: { allUsers: true } },
+    { $match: { "ns": /^mydb\./ } }
+  ],
+  cursor: {}
+})
+```
+
+## Key Takeaways
+
+- **Must be first stage** — `$currentOp` must be the first stage in the aggregation pipeline
+- **Admin database only** — this stage must be run against the `admin` database using `db.adminCommand()`
+- **Collection-agnostic** — does not operate on a specific collection

api-reference/operators/aggregation/$replaceroot.md

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+---
+title: $replaceRoot
+description: The $replaceRoot stage replaces the input document with the specified document.
+type: operators
+category: aggregation
+---
+
+# $replaceRoot
+
+The `$replaceRoot` stage replaces the input document with the specified document. The operation replaces all existing fields in the input document, including the `_id` field. This is useful for promoting an embedded document to the top level.
+
+## Syntax
+
+```javascript
+{
+  $replaceRoot: {
+    newRoot: <expression>
+  }
+}
+```
+
+## Parameters
+
+| Parameter | Description |
+| --- | --- |
+| **`newRoot`** | Required. A document expression that resolves to a document. The expression can be any valid expression that resolves to a document, such as a field path to an embedded document, a `$mergeObjects` expression, or a literal document. |
+
+## Examples
+
+Consider this sample document from the stores collection.
+
+```json
+{
+  "_id": "0fcc0bf0-ed18-4ab8-b558-9848e18058f4",
+  "name": "First Up Consultants | Beverage Shop - Satterfieldmouth",
+  "location": {
+    "lat": -89.2384,
+    "lon": -46.4012
+  },
+  "staff": {
+    "totalStaff": {
+      "fullTime": 8,
+      "partTime": 20
+    }
+  },
+  "sales": {
+    "totalSales": 75670,
+    "salesByCategory": [
+      { "categoryName": "Wine Accessories", "totalSales": 34440 }
+    ]
+  }
+}
+```
+
+### Example 1: Promote an embedded document
+
+Promote the `staff.totalStaff` subdocument to the top level:
+
+```javascript
+db.stores.aggregate([
+  { $replaceRoot: { newRoot: "$staff.totalStaff" } },
+  { $limit: 2 }
+])
+```
+
+This query returns:
+
+```json
+[
+  { "fullTime": 8, "partTime": 20 }
+]
+```
+
+### Example 2: Use $mergeObjects to combine fields
+
+Merge the staff subdocument with additional top-level fields:
+
+```javascript
+db.stores.aggregate([
+  {
+    $replaceRoot: {
+      newRoot: {
+        $mergeObjects: [
+          "$staff.totalStaff",
+          { storeName: "$name", totalSales: "$sales.totalSales" }
+        ]
+      }
+    }
+  },
+  { $limit: 2 }
+])
+```
+
+This query returns:
+
+```json
+[
+  {
+    "fullTime": 8,
+    "partTime": 20,
+    "storeName": "First Up Consultants | Beverage Shop - Satterfieldmouth",
+    "totalSales": 75670
+  }
+]
+```
+
+### Example 3: Replace root after $unwind
+
+Extract individual sales categories as top-level documents:
+
+```javascript
+db.stores.aggregate([
+  { $unwind: "$sales.salesByCategory" },
+  {
+    $replaceRoot: {
+      newRoot: {
+        $mergeObjects: [
+          "$sales.salesByCategory",
+          { storeName: "$name" }
+        ]
+      }
+    }
+  },
+  { $limit: 3 }
+])
+```
+
+## Limitations
+
+- If `newRoot` evaluates to a missing value or a non-document type, the operation errors
+
+## Key Takeaways
+
+- **Replaces the entire document** — the output document is the evaluated `newRoot` expression
+- **`$replaceWith` is an alias** — `{ $replaceWith: <expression> }` is shorthand for `{ $replaceRoot: { newRoot: <expression> } }`
+- **Combine with `$mergeObjects`** — use `$mergeObjects` to preserve fields from the original document while promoting an embedded document