docs(telemetry): add docs about standard instruments for subgraph

[bnjjj]

Added documentations about standard instruments available at the subgraph service:

• http.client.request.body.size - A histogram of request body sizes for requests handled by subgraphs.
• http.client.request.duration - A histogram of request durations for requests handled by subgraphs.
• http.client.response.body.size - A histogram of response body sizes for requests handled by subgraphs.

These instruments are configurable in router.yaml:

telemetry:
  instrumentation:
    instruments:
      subgraph:
        http.client.request.body.size: true # (default false)
        http.client.request.duration: true # (default false)
        http.client.response.body.size: true # (default false)

---

Checklist

Complete the checklist (and note appropriate exceptions) before the PR is marked ready-for-review.

• Changes are compatible¹
• Documentation² completed
• Performance impact assessed and acceptable
• Tests added and passing³
  • Unit Tests
  • Integration Tests
  • Manual Tests

Exceptions

Note any exceptions here

Notes

Footnotes

1. It may be appropriate to bring upcoming changes to the attention of other (impacted) groups. Please endeavour to do this before seeking PR approval. The mechanism for doing this will vary considerably, so use your judgement as to how and when to do this. ↩

2. Configuration is an important part of many changes. Where applicable please try to document configuration examples. ↩

3. Tick whichever testing boxes are applicable. If you are adding Manual Tests, please document the manual testing (extensively) in the Exceptions. ↩

[router-perf]

CI performance tests

• step - Basic stress test that steps up the number of users over time
• reload - Reload test over a long period of time at a constant rate of users
• step-with-prometheus - A copy of the step test with the Prometheus metrics exporter enabled
• events_without_dedup_callback - Stress test for events with a lot of users and deduplication DISABLED using callback mode
• no-tracing - Basic stress test, no tracing
• events - Stress test for events with a lot of users and deduplication ENABLED
• events_without_dedup - Stress test for events with a lot of users and deduplication DISABLED
• 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
• xlarge-request - Stress test with 10 MB request payload
• const - Basic stress test that runs with a constant number of users
• 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
• step-jemalloc-tuning - Clone of the basic stress test for jemalloc tuning
• xxlarge-request - Stress test with 100 MB request payload
• demand-control-uninstrumented - A copy of the step test, but with demand control monitoring enabled
• events_callback - Stress test for events with a lot of users and deduplication ENABLED in callback mode
• large-request - Stress test with a 1 MB request payload
• demand-control-instrumented - A copy of the step test, but with demand control monitoring and metrics enabled

[abernix]

Drive by comment. This is a well-prepared PR! I left one comment. It would be good for @BrynCooke and @Meschreiber to take a looksie beyond that.

[Meschreiber]

Left some preposition nits, otherwise LGTM!