Benchmarks: Create a guide on how to interpret daily benchmark results

[marclop]

Description

We should write a document that outlines how to interpret the numbers reported by the daily benchmarks. This could include.

• How different metrics are correlated and their potential meaning
• The purpose of each metric
• What to watch out for.
• Identifying flakyness
• Investigating a potential performance regression.

Writing this guide could also help in understanding how the APM Server works internally, common bottlenecks, pitfalls and general architecture.

[ericywl]

Gonna continue with this task, now that #17605 has been investigated and pending Fleet / Elastic Agent team to reply.