ssl session id and spdy/http2 traffic (no replies)

I have noticed that the variables ssl_session_id and ssl_session_reused are
always empty for http2 traffic (and for spdy before that). Under http 1.1
they are set as expected and documented.
What's the reason for this? Why not list the ID of the single connection
that is used for the multiplexing?
A big reason why I am logging both variables is to make sure that ssl
connections are being reused for http2 and to be able to check if the ssl cache
overruns. If the those two variables are not available, what do you suggest
to use instead?

Thanks!