Implement the ConnectionPoolStats in the AsyncClientFactory & expose them through appendStats, setting any stats (like refreshing) to 0 if they are not relevant. We will want to write a test similar to jstests/with_mongot/mongotmock/configure_mongot_connpool.js in order to test these stats, though it can be more straightforward as we don't care about refreshing stats or connection maximums, as our gRPC implementation uses one connection per remote anyways.