Report io thread metrics

[jlisam]

What does this PR do?

Since Redis 6 brings support for multi threaded IO, this PR adds support to redisdb to retrieve io_threads_active, io_threaded_writes_processed io_threaded_reads_processed from info and store them as metrics. These metrics will help determine the rate of I/O threads usage.

Motivation

We will be supporting Redis 6 in the near future and will need the new metrics piped to Datadog for evaluation and monitoring.

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