add prometheus metrics timestamp

[Betula-L]

This PR is to fix #2123

It includes,

• Update vendor prometheus client_golang
• Add exact scrape timestamp for each Prometheus counter metric

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[k8s-ci-robot]

Hi @Betula-L. Thanks for your PR.

I'm waiting for a google or 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.

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[Betula-L]

I signed it!

[googlebot]

CLAs look good, thanks!

[dashpole]

/ok-to-test

[dashpole]

looks like you will need to update the test data to include timestamps as well: https://github.com/google/cadvisor/tree/master/metrics/testdata

[Betula-L]

/retest

[Betula-L]

I forgot test. Finished now.

[dashpole]

Can you explain on the PR why the prometheus client version bump is required? Just for historical record. Looks good with one question.

[dashpole]

Why did these metrics disappear?

[Betula-L]

container_tasks_state is not removed. http_request_duration_microseconds is removed since it's not cAdvisor metrics, but prometheus metrics, you will not catch it in cAdvisor /metrics.

[Betula-L]

For prometheus client_golang 0.8.1, it does not provide metrics with timestamp due to prometheus/prometheus#398. The function NewMetricWithTimestamp is introduced in prometheus client_golang 0.9. A detailed issue track of that is in prometheus/client_golang#187

[dashpole]

lgtm