Add feature toggle for custom error pages /metrics

[ricardoapl]

What this PR does / why we need it:

Adds feature toggle to expose /metrics and /debug/vars endpoints from custom error pages default backend

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• CVE Report (Scanner found CVE and adding report)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation only

Which issue/s this PR fixes

Fixes #9152

How Has This Been Tested?

Tested manually on local machine by running the binary with different environment variable combinations such as

ERROR_FILES_PATH=./www ./app

curl --include 127.0.0.1:8080/
HTTP/1.1 404 Not Found
Content-Type: text/html
Date: Fri, 16 Feb 2024 11:29:18 GMT
Content-Length: 60

<span>The page you're looking for could not be found.</span>

curl --include 127.0.0.1:8080/metrics
HTTP/1.1 200 OK
Content-Type: text/plain; version=0.0.4; charset=utf-8
Date: Fri, 16 Feb 2024 11:30:10 GMT
Transfer-Encoding: chunked

# HELP default_http_backend_http_request_count_total Counter of HTTP requests made.
# TYPE default_http_backend_http_request_count_total counter
default_http_backend_http_request_count_total{proto="1.1"} 8
...

curl --include 127.0.0.1:8080/debug/vars
HTTP/1.1 200 OK
Content-Type: application/json; charset=utf-8
Date: Fri, 16 Feb 2024 11:31:25 GMT
Transfer-Encoding: chunked

{
"cmdline": ["./app"],
...

METRICS_PORT=8081 ERROR_FILES_PATH=./www ./app

curl --include 127.0.0.1:8080/
HTTP/1.1 404 Not Found
Content-Type: text/html
Date: Fri, 16 Feb 2024 11:33:09 GMT
Content-Length: 60

<span>The page you're looking for could not be found.</span>

curl --include 127.0.0.1:8080/metrics
HTTP/1.1 404 Not Found
Content-Type: text/html
Date: Fri, 16 Feb 2024 11:33:55 GMT
Content-Length: 60

<span>The page you're looking for could not be found.</span>

curl --include 127.0.0.1:8081/metrics
HTTP/1.1 200 OK
Content-Type: text/plain; version=0.0.4; charset=utf-8
Date: Fri, 16 Feb 2024 11:34:21 GMT
Transfer-Encoding: chunked

# HELP default_http_backend_http_request_count_total Counter of HTTP requests made.
# TYPE default_http_backend_http_request_count_total counter
...

IS_EXPORT_METRICS=false METRICS_PORT=8081 ERROR_FILES_PATH=./www ./app

curl --include 127.0.0.1:8080/        
HTTP/1.1 404 Not Found
Content-Type: text/html
Date: Fri, 16 Feb 2024 11:37:37 GMT
Content-Length: 60

<span>The page you're looking for could not be found.</span>

curl --include 127.0.0.1:8080/metrics
HTTP/1.1 404 Not Found
Content-Type: text/html
Date: Fri, 16 Feb 2024 11:37:49 GMT
Content-Length: 60

<span>The page you're looking for could not be found.

curl --include 127.0.0.1:8080/debug/vars
HTTP/1.1 404 Not Found
Content-Type: text/html
Date: Fri, 16 Feb 2024 11:40:47 GMT
Content-Length: 60

<span>The page you're looking for could not be found.</span>

curl --include 127.0.0.1:8081/metrics
curl: (7) Failed to connect to 127.0.0.1 port 8081 after 0 ms: Couldn't connect to server

curl --include 127.0.0.1:8081/debug/vars
curl: (7) Failed to connect to 127.0.0.1 port 8081 after 0 ms: Couldn't connect to server

IS_EXPORT_METRICS=false ERROR_FILES_PATH=./www ./app

curl --include 127.0.0.1:8080/       
HTTP/1.1 404 Not Found
Content-Type: text/html
Date: Fri, 16 Feb 2024 11:39:42 GMT
Content-Length: 60

<span>The page you're looking for could not be found.</span>

curl --include 127.0.0.1:8080/metrics
HTTP/1.1 404 Not Found
Content-Type: text/html
Date: Fri, 16 Feb 2024 11:39:59 GMT
Content-Length: 60

<span>The page you're looking for could not be found.</span>

curl --include 127.0.0.1:8080/debug/vars
HTTP/1.1 404 Not Found
Content-Type: text/html
Date: Fri, 16 Feb 2024 11:40:09 GMT
Content-Length: 60

<span>The page you're looking for could not be found.</span>

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have added unit and/or e2e tests to cover my changes.
• All new and existing tests passed.

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[k8s-ci-robot]

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If Ingress contributors determines this is a relevant issue, they will accept it by applying the triage/accepted label and provide further guidance.

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[k8s-ci-robot]

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[k8s-ci-robot]

Hi @ricardoapl. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[ricardoapl]

This is still missing changes to the Helm chart, but I would really appreciate some feedback before the implementation is complete

[ricardoapl]

/assign @tao12345666333

[tao12345666333]

/ok-to-test

[ricardoapl]

@tao12345666333 @strongjz please take another look, I believe I have resolved all of the issues

[tao12345666333]

Thanks! I will have a test today.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: ricardoapl
Once this PR has been reviewed and has the lgtm label, please ask for approval from tao12345666333. For more information see the Kubernetes Code Review Process.

The full list of commands accepted by this bot can be found here.

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[tao12345666333]

I have re-run the Helm chart lint job

[ricardoapl]

I have re-run the Helm chart lint job

Thank you @tao12345666333, I believe I have made a mistake in the Helm chart unit tests -- I will fix that

I will also try to add an e2e test for these changes as suggested by @strongjz on Slack

[ricardoapl]

I have fixed the Helm chart unit tests and added e2e tests as well

Please take another look @tao12345666333