Resolve "Add metrics for the size of the diff cache in redis"

What does this MR do?

Originally in !24224 (merged) we enabled the observing of diff caching size, but encountered an issue in production due to the command for checking the memory usage of a given key being added to Redis only in v4.0.0, however our production fleet was running v3.x (#202198 (closed))

Redis v5.0.x upgrade is in the works (#1708 (closed)) but even then, we may have existing customers who are running Redis 3.x. Until we officially deprecate our support of v3.x, we can still track offer memory usage for our v4.0.0+ installations by checking the version.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #37456 (closed)