The default-http-backend for handling 404 pages will now point to 404…

[vbannai]

… handler

with prometheus integration and provides metrics related to requests per second
and the duration of responding to the requests for various percentile
groupings. Please check
https://github.com/kubernetes/ingress-gce/blob/master/cmd/404-server-with-metrics/README.md
for details about the 404-server-with-metrics.

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind api-change
/kind bug
/kind cleanup
/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

The behavior of the default handler for 404 requests fro the GCE Ingress load balancer is slightly modified in the sense that it now exports metrics using prometheus. The metrics exported include: 
- http_404_request_total  (the number of 404 requests handled)
- http_404_request_duration_ms (the amount of time the server took to respond in ms)

Also includes percentile groupings. The directory for the default 404 handler includes instructions on how to enable prometheus for monitoring and setting alerts.

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[MrHohn]

/ok-to-test

[MrHohn]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: MrHohn, vbannai

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[MrHohn]

/kind feature
/priority important-soon

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