http3: reuse clients on RoundTripOpt context canceled

[GeorgeMac]

👋 Unsure if this is really a fix or not, but wanted to share for your consideration.

I found that when requests were getting context canceled for whatever reason (e.g. client going away) that the roundtripper was abandoning the connection and dialing again. This PR adds a check to observe the context canceled error and not remove the client in this particular instance. So that the cached client can be used in a subsequent round trip.

I appreciate context canceled can be a bit of a blunt instrument, so I am unsure if this is going to work in the broader state machine, but thought I would open it up for your thoughts.

p.s. love the project. I am attempting to build a reverse tunnel with, which is where I observed this behaviour:
https://github.com/flipt-io/reverst/. In my situation I am proxying a HTTP request from one server onto the http3 client. When e.g. my inbound request is canceled (e.g. the client goes away) this causes the connect to be dropped, which is quite a pain.

Update: I have included this in the project mentioned above and now the connection reuse is much better. I was getting constant reconnections without this change.

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[GeorgeMac]

I just signed the CLA. Unsure what needs doing to re-kick off that check. Is that something on your end, or does it happen automatically?

[marten-seemann]

Thank you for your PR and the thoughtful description.

The change lgtm, we want to remove connections that have already errored, not connections where the application decided to cancel (possibly immediately after dialing).

[GeorgeMac]

Thank you @marten-seemann 🙏

[marten-seemann]

Thanks for adding the explanation!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.75%. Comparing base (e48e1d4) to head (7a02178).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4448      +/-   ##
==========================================
+ Coverage   84.75%   84.75%   +0.01%     
==========================================
  Files         152      152              
  Lines       14371    14376       +5     
==========================================
+ Hits        12179    12184       +5     
  Misses       1691     1691              
  Partials      501      501              

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