Chore: Introduce new index to query active livechat conversations for…

… cloud scaling (#26047)

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Query to be used:
```
{ t: 'l', "source.type": 'widget',  open: true, "v.status": {$ne: 'offline'} } 
```
### Next steps
- Prometheus metric

apps/meteor/app/models/server/models/LivechatRooms.js

@@ -23,6 +23,16 @@ export class LivechatRooms extends Base {
 		this.tryEnsureIndex({ t: 1, departmentId: 1, closedAt: 1 }, { partialFilterExpression: { closedAt: { $exists: true } } });
 		this.tryEnsureIndex({ source: 1 }, { sparse: true });
 		this.tryEnsureIndex({ departmentAncestors: 1 }, { sparse: true });
+		this.tryEnsureIndex(
+			{ 't': 1, 'open': 1, 'source.type': 1, 'v.status': 1 },
+			{
+				partialFilterExpression: {
+					't': { $eq: 'l' },
+					'open': { $eq: true },
+					'source.type': { $eq: 'widget' },
+				},
+			},
+		);
 	}
 
 	findLivechat(filter = {}, offset = 0, limit = 20) {