Instrument counter metrics with CreatedTimestamp

[eriksywu]

address #3846

Various layers of the prom ecosystem now supports or are actively developing StartTimestamp/CreatedTimestamp awareness. CT values are natively supported by the prom-proto exposition format which the prom-go client supports.

make docker-test-integration
...
=== RUN   TestMetricsResponseContainsComments
--- PASS: TestMetricsResponseContainsComments (0.01s)
=== RUN   TestProtoCreatedTimestamps
--- PASS: TestProtoCreatedTimestamps (0.02s)

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.