http2: intercept what headers are being sent

[mcollina]

• Version: master, 8.8.1
• Platform: all
• Subsystem: http2

In http2 client API is not currently possible to know which headers are being sent. The actual headers that have been sent should be kept somewhere for debugging/introspection purposes.

[kasimdoctor]

@mcollina I would like to help on this. Is there a possibility of working with a mentor or someone who is familiar with this module?

[mcollina]

@jasnell @apapirovski @sebdeckers or myself. However just tag @nodejs/http2 in your PR.
You can also find me on node-dev on IRC.