Add count type to go_expvar

[hithwen]

Adds count metrics to go_expvar

[codecov]

Codecov Report

Merging #5097 into master will increase coverage by 2.08%.
The diff coverage is 100%.

Impacted Files	Coverage Δ
go_expvar/tests/test_unit.py	100% <100%> (ø)	⬆️
go_expvar/datadog_checks/go_expvar/go_expvar.py	82.44% <100%> (+0.13%)	⬆️
go_expvar/datadog_checks/go_expvar/__about__.py	100% <0%> (ø)	⬆️
go_expvar/datadog_checks/go_expvar/__init__.py	100% <0%> (ø)	⬆️
go_expvar/tests/common.py	100% <0%> (ø)	⬆️
go_expvar/tests/conftest.py	100% <0%> (ø)	⬆️
...checks_dev/tests/plugin/test_environment_runner.py
oracle/tests/test_oracle.py
...r/datadog_checks/openstack_controller/__about__.py
... and 832 more