Support more recent version of nginx-ingress that doesn't contain VTS exporter

This issue is related to: #41828 (closed)

Summary

Using more recent versions of nginx-ingress will not display certain stats on metrics screen because nginx-ingress excluded VTS in favor of other ways for prometheus implementation since version: 0.16.1

Now keys that are used in additional_metrics.yml don't match new naming of the keys that are being scraped by prometheus, more specifically - all keys defined under NGINX Ingress section also defined here : https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/user/project/integrations/prometheus_library/nginx_ingress.md

Keys in question: nginx_upstream_responses_total nginx_upstream_response_msecs_avg nginx_upstream_responses_total

Steps to reproduce

Install any ingress starting from 0.16.1, and access metrics in gitlab. it will have 4 system stats only, and no environment specific graphs are going to be visible (HTTP error rate, Latency, Throughput)

What is the current bug behavior?

No environment metrics available, only system specific metrics

What is the expected correct behavior?

Correct behavior is to support both - VTS and new style of stats in prometheus, all it needs really is just a separate section with new keys listed in there in place of old ones, although, because i'm not a current prometheus user - i'm a little confused as to which ones of the keys should be used, otherwise I'd make a pull request :)

Relevant logs and/or screenshots

(Paste any relevant logs - please use code blocks (```) to format console output, logs, and code as it's very hard to read otherwise.)

Results of GitLab environment info

11.1.4, omnibus and gitlab chart environments

Possible fixes

Need to add another group for nginx current, and rename existing to NGINX Ingress - Legacy