Don't record duration for failing requests.

What does this MR do?

If a request failed, we shouldn't care how fast it failed.

These recordings are used for the apdex aspect for error budget recordings for stage groups. With the current implementation, if a stage group has a single request that failed fast, they would have 50% availability. This should be 0% to match how we measure availability for the services themselves.

For those, we only measure apdex if the request succeeded: https://gitlab.com/gitlab-org/gitlab/blob/d969d80acb4bb416d2b28423de6810d314a3cfc8/lib/gitlab/metrics/requests_rack_middleware.rb#L82

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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