Collect version metadata

[coignetp]

What does this PR do?

Retrieve envoy version and set the associated metadata.

Motivation

Additional Notes

The version is available at http://domain/thing/server_info, so it uses the stats_url attribute in the config file http://domain/thing/stats and transforms it. So there is no additional attribute in the configuration file, but it does not cover some edge cases.

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 #6595 into master will increase coverage by 5.01%.
The diff coverage is 92.45%.

Impacted Files	Coverage Δ
envoy/datadog_checks/envoy/envoy.py	73.77% <80.95%> (+1.49%)	⬆️
envoy/tests/common.py	92.30% <100.00%> (+1.39%)	⬆️
envoy/tests/test_envoy.py	100.00% <100.00%> (ø)
cisco_aci/datadog_checks/cisco_aci/aci_metrics.py
kubelet/datadog_checks/kubelet/prometheus.py
kube_dns/tests/test_kube_dns.py
etcd/tests/conftest.py
cacti/datadog_checks/cacti/__init__.py
couch/tests/conftest.py
druid/datadog_checks/druid/__init__.py
... and 809 more

[ofek]

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