dynamic_modules: trigger downstream watermark callbacks when terminal

[anuraaga]

Commit Message: dynamic_modules: trigger downstream watermark callbacks when terminal

Additional Description:

Currently, dynamic modules have no way of knowing when the downstream buffers are filling to be able to apply response backpressure. This is notably important for terminal filters that actually generate the response.

On the flip side, non-terminal filters shouldn't need it. I considered making filters opt-in to the callbacks but felt the terminal filter knob may be enough of a signal and kept it simple. This comes from reading the flow control doc and its notion that the router filter opts into notifications to avoid "number of streams * number of filters" callbacks - I think terminal dynamic modules are the equivalent of router in this context. Let me know any thoughts.

envoy/source/docs/flow_control.md

Line 88 in 6ab1e7a

the downstream connection. It passes events to the router filter via

Risk Level: Low
Testing: Integration test
Docs Changes:
Release Notes: N/A
Platform Specific Features: N/A
Fixes #41432

/cc @mathetake @wbpcode

[anuraaga]

config_ can only be nullptr in some implementation detail unit tests and instead of making tests pass the expected config I just added the null check. Maybe my approach of registering the watermark callbacks implicitly like this isn't good after all, let me know

[mathetake]

@anuraaga super busy so let me get back to you next week