Measure Redis command latency per instance

What does this MR do?

gitlab-com/gl-infra/scalability#439 (closed)

This adds a prometheus histogram that lets us measure the latency of Redis calls, broken down by Redis instance.

The purpose of this is to be able to monitor apdex for each Redis instance (Cache, SharedState, Queues). The reason we do it here, on the client side, is that Redis itself does not expose the required latency metrics.

Screenshots

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