Execute the entire request pipeline if the client closed the connection

[Geal]

When the client closes the connection prematurely, it drops the entire request handling task, which means that it won't go through the entire response pipeline, where we record the operation and handle telemetry. Some users also have additional steps with rhai or coprocessors where they add metadata, and those steps should run even on canceled requests. This moves the request handling to a separate task to make sure it runs, but it also skips subgraph requests if we detected that the client closed the connection, to prevent unneeded traffic.

Description here

Fixes #issue_number

---

Checklist

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

• [ ] Changes are compatible[^1]
• [ ] Documentation[^2] completed
• [ ] Performance impact assessed and acceptable
• Tests added and passing[^3]
  • [ ] Unit Tests
  • [ ] Integration Tests
  • [ ] Manual Tests

Exceptions

Note any exceptions here

Notes

[^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.

[github-actions[bot]]

@Geal, please consider creating a changeset entry in /.changesets/. These instructions describe the process and tooling.

[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