When tried to use Mongolastic project noticed that there is no possiblity to apply Mongo collection changes to ElasticSearch index.
Therefore created a MongoToElasticObserver class, that transfers Mongo collection changes to ElasticSearch index.
Project is created with VS2019 and .NET 4.7.2.
Used NuGet packages: NEST, Newtonson, MongoDB.
This code can be used as an example, there are many things that can be improved here, but my goal was to share only idea.