I would appreciate you'll try to solve the pain of most users using mongodb in a serverless framework:

The cold start problem of the first connection to mongo-db using Lambda.

The idea - to build a proxy that will serve multiple and different lambdas, while the proxy itself will be implemented using a Lambda function as well. 

Please refer to RDS Proxy by AWS that solved the same issue for relational databases.

The point is to keep the given client as is with an additional layer of connecting to a proxy layer that actually holds the client and can be warmed up in advance. 

It's a win-win situation: 

1. Using a proxy we can reduce costs significantly.

2. We make the serverless approach using Rust much more attractive with MongoDB.

3. We reduce cold starts.