Summary

There seems to be a bug in how the query is translated when using AsQueryable().Last() or AsQueryable().LastOrDefault(). It will fail with an exception: `FormatException: Element '_last' does not match any field or property of class MyClass.`

If we look at the code, you can see that it uses an aggregation:

```csharp
pipeline = pipeline.AddStage(
AstStage.Group(
id: BsonNull.Value,
fields: AstExpression.AccumulatorField("_last", AstUnaryAccumulatorOperator.Last, AstExpression.RootVar)),
pipeline.OutputSerializer);
```

So the last object of the collection ends up wrapped inside a "_last" field of another object.

Looks like you need to add another stage here to return the object itself without the wrapper:
```csharp
AstStage.ReplaceRoot(AstExpression.GetField(AstExpression.RootVar, "_last"))
```

How to Reproduce

Connect to the server using MongoDB.Driver and try executing AsQueryable().Last() on a collection.

Additional Information

Looks like an easy fix to me. I could make a PR.