MongoDB extractor

Hi,

We're using the Cognite DB extractor (version 3.9.2) with a MongoDB database (Azure Cosmos DB via the MongoDB API) and are trying to set up incremental loading.

The documentation for mongodb (https://docs.cognite.com/cdf/integration/guides/extraction/configuration/db#databases.mongodb) doesn't mention the use of start_at and incremental_field but after testing them and looking at the debug logs, we can see the extractor sends the {start_at} placeholder literally without substituting the state value. 
My questions are:
    1. Is start_at substitution supported at all for MongoDB JSON queries, or is it SQL-only?
    2. If incremental loading is not supported for MongoDB, is there a recommended workaround?

Thanks.