[docs] Add docs about span macros

[smyrick]

The docs reference the macros available but only mention how to use the metric ones when we do support the span macros too

[router-perf[bot]]

CI performance tests

• [ ] reload - Reload test over a long period of time at a constant rate of users
• [ ] events_big_cap_high_rate_callback - Stress test for events with a lot of users, deduplication enabled and high rate event with a big queue capacity using callback mode
• [ ] events_without_dedup_callback - Stress test for events with a lot of users and deduplication DISABLED using callback mode
• [ ] large-request - Stress test with a 1 MB request payload
• [x] const - Basic stress test that runs with a constant number of users
• [ ] no-graphos - Basic stress test, no GraphOS.
• [ ] step-jemalloc-tuning - Clone of the basic stress test for jemalloc tuning
• [ ] events - Stress test for events with a lot of users and deduplication ENABLED
• [ ] events_callback - Stress test for events with a lot of users and deduplication ENABLED in callback mode
• [ ] events_big_cap_high_rate - Stress test for events with a lot of users, deduplication enabled and high rate event with a big queue capacity
• [ ] events_without_dedup - Stress test for events with a lot of users and deduplication DISABLED
• [ ] xxlarge-request - Stress test with 100 MB request payload
• [ ] xlarge-request - Stress test with 10 MB request payload
• [x] step - Basic stress test that steps up the number of users over time

[abernix]

@shorgi Could you re-review? Thanks!