user-agent addition to libhoney client doesn't work anymore

[robbkidd]

Libhoney v1.19.0 added the experimental transmission and changed the timing of transmission initialization. Prior to that, a libhoney client would initialize its transmission upon sending the first event. That gave the Beeline the apportunity to manipulate the user_agent_addition private instance variable in the libhoney client in a race to catch it after client instantiation and before the first event is sent. Libhoney v1.19.0's introduction of a new transmission type also changed the timing of transmission init to libhoney client initialization.

The Beeline ought to use only the libhoney client initialization parameter to add itself to the user_agent string. This will mean that users who instantiate a libhoney client to provide to the Beeline (for example, to configure web proxy settings) will not have the Beeline mentioned in the user_agent when sending events, despite the use of the Beeline. This is sad, but also consistent with the behavior of the other language Beelines, none of which manipulate a libhoney transmission's user_agent when provided with a user-instantiated libhoney/transmission.