add active defragmentation gauges for redis

[tanner-bruce]

What does this PR do?

The active defrag gauges are missing from the redis integration and will be useful for monitoring the effects on CPU usage and overhead while active defrag is running

Motivation

Investigating active defragmentation and would like to view the metrics supplied by redis in DataDog

Additional Notes

Adds a few standard redis metrics from the INFO call

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

[tanner-bruce]

@hithwen thanks for the review, I've updated the PR now.

[codecov]

Codecov Report

Merging #5022 into master will decrease coverage by 10.03%.
The diff coverage is 0%.

Impacted Files	Coverage Δ
redisdb/datadog_checks/redisdb/redisdb.py	73.82% <ø> (ø)	⬆️
redisdb/tests/test_e2e.py	10.97% <0%> (-1.53%)	⬇️
redisdb/tests/conftest.py	85.71% <0%> (-4.77%)	⬇️
redisdb/tests/test_default.py	95.76% <0%> (ø)	⬆️
redisdb/tests/test_unit.py	100% <0%> (ø)	⬆️
redisdb/tests/common.py	100% <0%> (ø)	⬆️
redisdb/datadog_checks/redisdb/__about__.py	100% <0%> (ø)	⬆️
redisdb/tests/test_replication.py	100% <0%> (ø)	⬆️
redisdb/tests/test_auth.py	92.59% <0%> (ø)	⬆️
... and 826 more