updating prometheus metrics names according to naming best practices

[noqcks]

What this PR does / why we need it: This fixes #1509. We want to be following best practices! https://prometheus.io/docs/practices/naming/

Which issue(s) this PR fixes:
Fixes #1509

Special notes for your reviewer:

Release note:

Updated Nginx metrics names according to Prometheus best practices

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[k8s-reviewable]

This change is 

[aledbf]

@noqcks thank you for doing this

[aledbf]

@noqcks please sign the CLA

[noqcks]

Just signed CLA. Commenting to update.

[coveralls]

Coverage increased (+0.05%) to 36.691% when pulling b55fb6c on noqcks:adding-prometheus-metrics-best-practices into a1afc41 on kubernetes:master.

[aledbf]

/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, noqcks

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[aledbf]

@noqcks thanks!

[discordianfish]

Only counters should have _total suffixes, not gauges. Also keep in mind that changing these metrics might break alerts and dashboards, so these changes should be kept to a minimum.

@brancz: Would it be possible to have a tag or something for "includes metrics" that could be added to PRs that should get reviewed by someone with prometheus experience?

[brancz]

As this repo is under the Kubernetes org, we can simply use @kubernetes/sig-instrumentation-pr-reviews to tag the team that is aware of changes that involve instrumentation. Please do encourage anyone working on instrumentation related things to tag those.

Otherwise what @discordianfish said regarding counter/gauge naming is correct, and we should oblige to it.

[aledbf]

@discordianfish @brancz can you send a PR to fix counter/gauge naming issue?