Metrics: Add --metrics-per-undefined-host argument.

[grounded042]

What this PR does / why we need it:

This PR fixes #3844 by adding a flag, --metrics-per-undefined-host which
allows host labels on request metrics even if the host is not defined on an
ingress.

Today when a request metric comes in it will be dropped if the host label is
not a host explicitly defined on an ingress which ingress-nginx is tracking.
Even if --metrics-per-host is set to true it would drop metrics. You could
set --metrics-per-host to false, but then the host label was not included
in the metrics. The reasoning for this was due to wildcard hosts. Someone could
send many requests with different hostnames not defined on any ingress and the
cardinality of request metrics would explode due to the many hosts. This PR
gives those who accept the cardinality explosion risk a way to get metrics with
the host label included even when the host is not defined on an ingress.

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 #3844

How Has This Been Tested?

I have tested this in multiple ways:

• smoke testing in a make dev-env kind cluster. I ran the code before
  and after my change and checked metrics to see the metrics dropped before
  and not dropped after.
• added unit tests to the collector
• added unit tests to the flags
• added e2e tests

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.

[k8s-ci-robot]

Hi @grounded042. 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.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[netlify]

✅ Deploy Preview for kubernetes-ingress-nginx ready!

Name	Link
🔨 Latest commit	f872474
🔍 Latest deploy log	https://app.netlify.com/sites/kubernetes-ingress-nginx/deploys/66cc8fea5704c2000873796a
😎 Deploy Preview	https://deploy-preview-11818--kubernetes-ingress-nginx.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[Gacko]

/cc @strongjz
/uncc

[strongjz]

I meant to look at this Sunday. I will tonight.

[grounded042]

I've rebased the branch so it's ready to merge.

[strongjz]

/kind feature
/ok-to-test
/triage accepted
/priority backlog

[strongjz]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: grounded042, strongjz

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

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [strongjz]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment