Set default failure codes 5xx

[xxx7xxxx]

Fix #689

[localvar]

please update the document to mention the default values: https://github.com/megaease/easegress/blob/main/doc/reference/filters.md#proxyserverpoolspec

[codecov-commenter]

Codecov Report

Merging #757 (87043e3) into main (ea86a0e) will decrease coverage by 0.08%.
The diff coverage is 30.00%.

@@            Coverage Diff             @@
##             main     #757      +/-   ##
==========================================
- Coverage   80.44%   80.35%   -0.09%     
==========================================
  Files          97       97              
  Lines       10790    10798       +8     
==========================================
- Hits         8680     8677       -3     
- Misses       1623     1631       +8     
- Partials      487      490       +3     

Impacted Files	Coverage Δ
pkg/object/meshcontroller/spec/spec.go	68.96% <ø> (ø)
pkg/filters/proxy/pool.go	76.00% <30.00%> (-0.74%)	⬇️
pkg/cluster/cluster.go	60.10% <0.00%> (-1.06%)	⬇️

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[suchen-sci]

should we keep a way to not set failureCode? since failureCode is a result of proxy and may influence the execution of pipeline...

[xxx7xxxx]

please update the document to mention the default values

Document updated.

should we keep a way to not set failureCode? since failureCode is a result of proxy and may influence the execution of pipeline...

Yes, it's very annoying to manually set it one by one e.g [500, 501, 502...]. Because most time 5xx stands for server error, which would be a failure indication for circuit breaker and retry. [1]

[1] https://en.wikipedia.org/wiki/List_of_HTTP_status_codes#5xx_server_errors