Better metric names handling when the namespace is empty

[ahmed-mez]

What does this PR do?

• Remove the leading dot in metric names when the namespace is empty (e.g .process.vm.bytes -> process.vm.bytes)

Motivation

• Users might want to pass an empty namespace to the openmetrics check to ease the migration from Prometheus (by having the same metric names in DD)

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 #7567 into master will increase coverage by 0.72%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
redisdb/tests/test_auth.py
redisdb/tests/test_replication.py
go_expvar/tests/conftest.py
gitlab_runner/tests/test_unit.py
redisdb/tests/common.py
...ab_runner/datadog_checks/gitlab_runner/__init__.py
pdh_check/datadog_checks/pdh_check/__about__.py
pdh_check/tests/test_pdh_check.py
redisdb/tests/conftest.py
haproxy/tests/test_unit.py
... and 100 more