Add the ability to set the hits_addend for a given rate_limit request via a hardcoded dynamic metadata field

[EItanya]

Commit Message: Adds the ability to set the hits_addend for a given rate_limit request via a hardcoded dynamic metadata field: envoy.ratelimit:hits_addend.
Additional Description:
Risk Level: Low
Testing: Added unit test. I have also manually tested this using gloo-edge as the control-plane.
Docs Changes:
Release Notes:
Platform Specific Features: N/A
[Optional Runtime guard:] N/A
[Optional Fixes #Issue] N/A
[Optional Fixes commit #PR or SHA] N/A
[Optional Deprecated:] N/A
[Optional API Considerations:] N/A

[repokitteh-read-only]

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Please mark your PR as ready when you want it to be reviewed!

🐱

Caused by: #34184 was opened by EItanya.

see: more, trace.

[EItanya]

The changes to ratelimit.cc are meant mostly as a conversation starter. I originally debated adding a MetadataKey to the Route spec to be able to generically specify a metadata key, but that felt overkill for such a small feature so I decided to start here. I can also of course make envoy.ratelimit a constant a la envoy.lb, but I wasn't sure of the appetite for that.

I'm also not sure exactly where the docs should live for this feature. I definitely think that updating the release notes make sense and I can do that, but it could definitely also make sense to add it to the proper docs.

[mattklein123]

At a high level this seems fine to me. For documentation I would add to the filter specific documentation as well as the release notes.

/wait

[EItanya]

Ok, I'll add the documentation, are there any code changes you'd like me to make while I'm at it?

[repokitteh-read-only]

CC @envoyproxy/api-shepherds: Your approval is needed for changes made to (api/envoy/|docs/root/api-docs/).
envoyproxy/api-shepherds assignee is @mattklein123
CC @envoyproxy/api-watchers: FYI only for changes made to (api/envoy/|docs/root/api-docs/).

🐱

Caused by: #34184 was synchronize by EItanya.

see: more, trace.

[jmarantz]

/retest

[jmarantz]

format failure

/wait