Fix version metadata collection

[coignetp]

What does this PR do?

Fix how we submit the nginx version metadata.

Send it only when the configuration enables it
Send the correct nginx version:

• From the header response in the classic API
• version from the /nginx endpoint of the PLUS API https://docs.nginx.com/nginx/admin-guide/monitoring/live-activity-monitoring/#getting-statistics-with-the-api, that it different from the API version, also named version in a different endpoint.

We explicitly get the version from the correct endpoint, so it's not override in the metric loop.

Motivation

Additional Notes

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached

[codecov]

Codecov Report

Merging #7972 (370c8cf) into master (f67c7b6) will increase coverage by 5.73%.
The diff coverage is 91.66%.

Impacted Files	Coverage Δ
nginx/datadog_checks/nginx/nginx.py	90.25% <75.00%> (+2.42%)	⬆️
nginx/tests/test_nginx.py	95.83% <100.00%> (+1.89%)	⬆️
redisdb/datadog_checks/redisdb/__init__.py
coredns/tests/test_check.py
marklogic/datadog_checks/marklogic/api.py
...er/datadog_checks/kube_scheduler/kube_scheduler.py
lighttpd/tests/common.py
squid/tests/common.py
airflow/tests/test_integration_e2e.py
ceph/datadog_checks/ceph/__about__.py
... and 957 more

[hithwen]

What's vts?

[coignetp]

A nginx module for virtual traffic (https://github.com/vozlt/nginx-module-vts)

[hithwen]

Send it only when the configuration enables it
Which change is doing this?

[ofek]

💯