Log bytes read from / written to Redis

What does this MR do?

This MR adds instrumentation for the number of bytes written to and read from Redis. Counting happens at the driver level. It includes the size of the commands, but excludes the size overhead of Redis serialization (length prefix, trailing \r\n, etc.).

This is part of gitlab-com/gl-infra/scalability#318 (closed).

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