Refactor check and support new Agent signature

[ChristineTChen]

Clean up constants and metrics in Istio check, refactor code

[codecov]

Codecov Report

Merging #6341 into master will increase coverage by 5.10%.
The diff coverage is 98.13%.

Impacted Files	Coverage Δ
istio/tests/conftest.py	85.54% <96.61%> (+27.20%)	⬆️
istio/datadog_checks/istio/constants.py	100.00% <100.00%> (ø)
istio/datadog_checks/istio/istio.py	96.47% <100.00%> (-0.24%)	⬇️
istio/datadog_checks/istio/metrics.py	100.00% <100.00%> (ø)
istio/tests/common.py	100.00% <100.00%> (ø)
istio/tests/test_check.py	100.00% <100.00%> (+1.98%)	⬆️
..._checks_base/datadog_checks/base/utils/tailfile.py
consul/datadog_checks/consul/consul.py
...tadog_checks/base/checks/kube_leader/base_check.py
prometheus/datadog_checks/prometheus/__about__.py
... and 834 more