Restore HTTP payload size limit, make it configurable

[SimonSapin]

Fixes #2000

Configuration:

limits:
  experimental_http_max_request_bytes: 2000000 # Default value: 2 MB

Checklist

Complete the checklist (and note appropriate exceptions) before a final PR is raised.

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

Exceptions

This change could be a compatibility issue for users who rely on large requests and upgrading from a Router version that didn’t have this limit. However this issue is easily fixed through configuration. The error log (but not the HTTP response) mention the relevant configuration key.

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.
- please raise a separate issue to automate the test and label it (or ask for it to be labeled) as manual test

[router-perf]

CI performance tests

• const - Basic stress test that runs with a constant number of users
• no-graphos - Basic stress test, no GraphOS.
• 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
• large-request - Basic stress test, no GraphOS.

[SimonSapin]

Open question: config location

I put this in the preview_operation_limits config section because that’s where we have other config keys that are most similar. However it’s not quite accurate since this new limit is about the entire HTTP body, not just operations. You could exceed it with a small operation and a large variable.

One way to resolve this might be, when moving this set of features out of preview, rename the section from preview_operation_limits to limits instead of to operation_limits.

[SimonSapin]

@chandrikas in case you want to have a look at the wording here. (The same is copied in the changelog.)

[chandrikas]

Thanks @SimonSapin. Made some minor edits but looks good overall