Some of the C++ integration tests use ConnectionString::connect to produce a DBClientBase, but the default behavior of this is to use DBClientConnection (a mongorpc-only type) unless gRPC is specified via the MongoURI argument. gRPC is specified trhough the --enableEgressGRPC argument to the integration tests rather than through the connection string though, so this results in a DBClientConnection being backed by a gRPC stream.

To fix this, we should ensure that gRPC is specified when calling ConnectionString::connect. The easiest way forward here might be to parse the --connectionString argument in integration tests with MongoURI, which supports the gRPC URI option. We may want to use that instead of a separate --useEgressGRPC argument.

To catch this in the future, we should also explicitly grab the MongoRPC transport layer in DBClientConnection, and invariant that one is present when doing so.