document techniques for redis traffic analysis and cpu profiling

As part of https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/9420 (and related redis work) we've developed some new analysis techniques.

One of them is more advanced traffic analysis. redis-trace-cmd.rb will process tcpflow output (derived from pcap files) into a text based format that also lends itself to keyspace pattern analysis (currently loaded with patterns for redis-cache).

The other one is CPU profiling. Flamescope can be used to dig into bursts on a sub-second timescale. Flamegraphs can give hints regarding workload changes. They can also highlight potential improvements to be made upstream (one example).