[Metrics] Add request error metrics

[JeffLuoo]

This change defines some general errors, the list might grow in the future if more finer error types are needed.

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[JeffLuoo]

@liu-cong @courageJ tag for review

[ahg-g]

/ok-to-test

If you don't mind, I would like to getting #271 in first since it is required for the release.

[ahg-g]

To fix the test error, please remove gci from the list of linters: https://github.com/kubernetes-sigs/gateway-api-inference-extension/blob/main/.golangci.yml#L17

[liu-cong]

Can you add an integration test for this? The code path is pretty complex now, especially due to the streaming processing, an integration test can give us higher confidence.

See #326. Feel free to defer to a follow up change.

[ahg-g]

Sorry, one more rebase, and this should be good to go.

[ahg-g]

Can we track metrics probing errors too? the probing that the provider does to get server load.

[JeffLuoo]

@ahg-g Can you provide more context on the question? Do you mean tracking errors when:

• EPP server fails to service /metrics endpoint
• Monitoring solutions (Prometheus or GMP) fail to scrape the /metrics endpoint of EPP

[liu-cong]

Can we track metrics probing errors too? the probing that the provider does to get server load.

Yeah we should add these, but I recommend in a follow up. This is tracked in #50 (comment)

[liu-cong]

/lgtm

[ahg-g]

/approve

Thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahg-g, JeffLuoo

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