refactor: add qlog factory, move group_id to common_fields

[Frando]

Description

This PR changes the qlog configuration such that on the endpoint level, users may supply a QlogFactory, which is invoked for each connection to (optionally) create a QlogConfig for that connection. This means that the usual behavior is now to write a qlog file per connection instead of a single file per endpoint.

This simplifies the operation in quinn_proto: Because a QlogSink is now per connection, we don't have to pass the group_id (which is the initial_dst_cid) for each emitted event, and can instead set it in the so-called CommonFields property of the qlog trace (which allows to set a field for each event in the trace). This will significantly reduce the file size of traces also, and is the recommended practice by the spec.

It will also enable apps to support the QLOGDIR environment variable, so that an endpoint creates a new file for each connection in that directory.

Background

Currently, quinn impls qlog support such that there is one QlogSink per endpoint, shared between all connections. The QlogSink forwards to a qlog::QlogStreamer, which writes events in the JSON-SEQ format. While the events have a group_id field that allows to disambiguate events from different connections, a qlog file in JSON-SEQ format only has a single qlog trace. And according to the spec, a trace logically maps to a single connection. The file in JSON-SEQ format does not allow for multiple traces. The trace-level metadata includes a so-called "vantage point", which is the observed role, so either client or server - a property that makes only sense per connection, not per endpoint.

Breaking Changes

• removed: TransportConfig::qlog_sink
• added: TransportConfig::qlog_factory

Notes & open questions

• With this change, we no longer have a place to emit qlog events unrelated to a connection. However, we currently do not emit any events that don't correlate directly to a connection. Should we happen to add such events in the future, we can extend the QlogFactory trait to create a config for such events.
• I'm not too keen on the Factory name, but also couldn't come up with better name .

[codecov-commenter]

Codecov Report

❌ Patch coverage is 21.10553% with 157 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.81%. Comparing base (fb79e91) to head (47880d5).

Files with missing lines	Patch %	Lines
quinn-proto/src/config/qlog.rs	0.00%	85 Missing ⚠️
quinn-proto/src/connection/qlog.rs	7.01%	53 Missing ⚠️
quinn-proto/src/config/transport.rs	56.66%	13 Missing ⚠️
perf/src/lib.rs	0.00%	4 Missing ⚠️
quinn-proto/src/connection/mod.rs	88.88%	2 Missing ⚠️

Additional details and impacted files

@@                       Coverage Diff                        @@
##           Frando/qlog-latest-multipath     #218      +/-   ##
================================================================
- Coverage                         76.01%   75.81%   -0.20%     
================================================================
  Files                                83       84       +1     
  Lines                             23019    23053      +34     
================================================================
- Hits                              17498    17478      -20     
- Misses                             5521     5575      +54     

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[Arqu]

👏

[flub]

• With this change, we no longer have a place to emit qlog events unrelated to a connection. However, we currently do not emit any events that don't correlate directly to a connection. Should we happen to add such events in the future, we can extend the QlogFactory trait to create a config for such events.

Having a look at https://quicwg.org/qlog/draft-ietf-quic-qlog-quic-events.html I could only find the server_listening event? Doesn't seem terribly important I guess. Though surely it's possible to add an endpoint-wide file later too?

[Frando]

Though surely it's possible to add an endpoint-wide file later too?
Yes, that was my thinking too. We can add more functions to the QlogFactory trait for that.