Make Gateway return 504 when HttpClient times out

[mmanciop]

When the underpinning Netty HttpClient times out (based, e.g., on the value of spring.cloud.gateway.httpclient.responseTimeout), the Gateway returns 500 to its client instead of 504.

This change wraps the org.springframework.cloud.gateway.support.TimeoutException thrown upon the timeout with a org.springframework.web.server.ResponseStatusException that sets the status code to 504.

As an added boon, using org.springframework.web.server.ResponseStatusException this way enables the usual error handling mechanisms (error white-page, error attributes) for this type of failure.

[mmanciop]

Technically, here one may say "Hey, why don't we get rid of TimeoutException altogether then"?

The answer is: it is a very convenient "marker" exception, which I already make use of in #555. I like the fact that, by not reusing java.util.concurrent.TimeoutException, one can tell apart this type of timeout from one issued, for example, by a filter using java.util.concurrent.Future for whatever dastardly reasons.

[codecov-io]

Codecov Report

Merging #558 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #558      +/-   ##
============================================
+ Coverage     70.69%   70.69%   +<.01%     
  Complexity      930      930              
============================================
  Files           129      129              
  Lines          3672     3662      -10     
  Branches        263      263              
============================================
- Hits           2596     2589       -7     
+ Misses          938      935       -3     
  Partials        138      138

Impacted Files	Coverage Δ	Complexity Δ
...ework/cloud/gateway/filter/NettyRoutingFilter.java	94.73% <100%> (ø)	15 <1> (+1)	⬆️
...y/handler/predicate/PathRoutePredicateFactory.java	76.66% <0%> (-1.12%)	8% <0%> (ø)
...cloud/gateway/config/GatewayAutoConfiguration.java	76.31% <0%> (-0.41%)	59% <0%> (-1%)
...ork/cloud/gateway/route/builder/PredicateSpec.java	72.34% <0%> (+2.95%)	28% <0%> (ø)	⬇️

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[mmanciop]

Similarly to #557, I moved the PR to 2.x . Do we want to treat this as a breaking change and target only the next release (G... Grubsomething? :D). If we want to ship it only with the next release, I'll gladly move this back to master.

[mmanciop]

Nevermind, it is a breaking change due to the change in status code. Well-behaving REST clients should not break because of a change from 500 to 504, but it would not be the first nor last time this happens in the wild.