Add 'redis.net.connections' metric to count connections by client

[remicalixte]

What does this PR do?

Adds a new metric to the redisdb check counting the number of connection by client name

Motivation

We have a internal check at datadog reporting this metric and we think it could be useful to a lot of people

Additional Notes

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 #6495 into master will decrease coverage by 1.27%.
The diff coverage is 100.00%.

Impacted Files	Coverage Δ
redisdb/datadog_checks/redisdb/redisdb.py	76.01% <100.00%> (ø)
redisdb/tests/test_default.py	96.57% <100.00%> (ø)
...cks/dev/tooling/configuration/consumers/example.py
tcp_check/tests/common.py
ignite/datadog_checks/ignite/__init__.py
ibm_db2/datadog_checks/ibm_db2/utils.py
kube_apiserver_metrics/tests/common.py
network/datadog_checks/network/network.py
prometheus/datadog_checks/prometheus/prometheus.py
...g_checks_base/datadog_checks/base/utils/tagging.py
... and 825 more

[remicalixte]

I think the CI is failing because I opened the PR from a fork

[florimondmanca]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[florimondmanca]

Great stuff, thanks for submitting this! I only have a few nits. :-)

[hithwen]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[florimondmanca]

Thank you!

[remicalixte]

Feel free to merge the PR as I don't have the permissions myself since the label job is failing