Add go_expvar.memstats.total_alloc.count

[ChristineTChen]

Previous PR to change total_alloc was reverted because of possible impact to go_expvar.memstats.total_alloc graph oh dashboards (#5972).

This PR allows us to keep compatibility while also submitting the metric correctly as a monotonic count under a new metric name go_expvar.memstats.total_alloc.count

[codecov]

Codecov Report

Merging #6005 into master will increase coverage by 4.41%.
The diff coverage is 100%.

Impacted Files	Coverage Δ
go_expvar/tests/common.py	100% <100%> (ø)	⬆️
go_expvar/tests/test_unit.py	100% <100%> (ø)	⬆️
go_expvar/datadog_checks/go_expvar/go_expvar.py	82.83% <100%> (+0.39%)	⬆️
go_expvar/tests/test_integration.py	100% <100%> (ø)	⬆️
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/conftest.py	100% <0%> (ø)	⬆️
... and 901 more