You can pass along timing data from every aspect of the request [1] currently there are approximately 20 events supported. You can also send over just about anything that can be stored within an HAProxy variable / header through a tag. Here's an example configuration for HAProxy (simple) [2] (advanced) [3] and the OpenTracing specific configuration (simple) [4] (advanced) [5]. I've included both simple and advanced configurations.

[1] https://github.com/haproxy/haproxy/blob/master/addons/ot/REA...

[2] https://github.com/haproxy/haproxy/blob/master/addons/ot/tes...

[3] https://github.com/haproxy/haproxy/blob/master/addons/ot/tes...

[4] https://github.com/haproxy/haproxy/blob/master/addons/ot/tes...

[5] https://github.com/haproxy/haproxy/blob/master/addons/ot/tes...