logging both http- and quic-level events

[kazuho]

At the moment, the command is capable of only logging either of the two at once. This prevents us from doing cross-layer analysis, e.g, looking into what HTTP response is sent on a particular QUIC STREAM frame.

I think we should simply log all the events regardless of the layer (e.g., provider in terms of USDT).

When emitting events from multiple providers to JSON, the event names can be prefixed by their prefixes (e.g., receive event of quicly provider becomes quicly_receive). Doing so would align the output to that generated by https://github.com/h2o/quicly/blob/master/misc/probe2trace.pl.

[toru]

Thank you for the feature request. This sounds like a lovely way to support the core development of H2O, as well as empowering system operators to make informed tuning decisions.

[kazuho]

See also #83.