The ToString method of ServerDescription purposely omits some of the fields of the class in order to keep the string length reasonable.

However, it would probably be helpful to include the LastUpdateTimestamp (at the very least when there is also a HeartbeatException).

The value of LastUpdateTimestamp should indicate when the last heartbeat completed (either successfully or in error). Knowing how long ago the last heartbeat completed should be very helpful in diagnosing connectivity issues.