http2: log pings and RoundTrip retries when http2debug=1
http2: log pings and RoundTrip retries when http2debug=1

Change-Id: I3a7358db65ebc474712817a776d8468d0b09f0ab
Reviewed-on: https://go-review.googlesource.com/c/net/+/359914
Trust: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
Run-TryBot: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
Trust: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/net