fix(kuma-cp) allow only one healthcheck

[jakubdyszkiewicz]

Summary

https://github.com/envoyproxy/envoy/blame/f4a88f969cfb58b36efcf649fc25e5c85e7e9f5c/source/common/upstream/cluster_factory_impl.cc#L124
Only one item in the Envoy health check list is allowed therefore we need extra validation until this is changed.

Issues resolved

Fix #2966

Documentation

• docs(policies) healthcheck can only have one check kuma-website#564

Testing

• Unit tests
• E2E tests
• Manual testing on Universal
• Manual testing on Kubernetes

Backwards compatibility

• Add backport-to-stable label if the code is backwards compatible. Otherwise, list breaking changes.

[michaelbeaumont]

LGTM

[codecov-commenter]

Codecov Report

Merging #2972 (143c366) into master (c586219) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2972      +/-   ##
==========================================
+ Coverage   52.37%   52.40%   +0.02%     
==========================================
  Files         912      912              
  Lines       52522    52524       +2     
==========================================
+ Hits        27510    27524      +14     
+ Misses      22817    22803      -14     
- Partials     2195     2197       +2     

Impacted Files	Coverage Δ
...core/resources/apis/mesh/health_check_validator.go	100.00% <100.00%> (ø)
pkg/core/resources/manager/cache.go	85.71% <0.00%> (-2.60%)	⬇️
pkg/xds/generator/direct_access_proxy_generator.go	83.90% <0.00%> (+1.14%)	⬆️
pkg/dns/vips_allocator.go	74.00% <0.00%> (+1.33%)	⬆️
pkg/plugins/runtime/gateway/route/sorter.go	66.66% <0.00%> (+5.12%)	⬆️
pkg/core/bootstrap/autoconfig.go	54.46% <0.00%> (+8.03%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c586219...143c366. Read the comment docs.