Improve HTTP method label handling in prometheus metrics

[jacekn]

What this PR does / why we need it:

This PR fixes #10208 by checking whether the request method is valid. For invalid methods we set method="invalid_method" label so that operators can stil see traffic in the metrics but without unbound label value.

Types of changes

• [x] 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

How Has This Been Tested?

I tested by building the image locally using make build && make image and running the image in minikube. The change has a fairly narrow scope so should be low risk.

Checklist:

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