api: adds hits_addend specifier to RateLimitRule

[mathetake]

What type of PR is this?

This adds a new API into rate limit API.

What this PR does / why we need it:

This is for #4756. Basically, this adds the API to specify the hits_addend for the rate limit rule.
Especially, configuring the hits_addend in the response_path allows us to "reduce" the counter
based on the response content that affects the subsequent requests. This will enable the "token"
based request limits that are required fro AI gateway.

This is based on the Envoy's brand new features called apply_on_stream_done
and per-descriptor level hits_addend configuration introduced in

• http ratelimit: option to reduce budget on stream done  envoy#37548
• global rate limit: supported ratelimits in the typed per filter config envoy#37684

Which issue(s) this PR fixes:
#4756

Release Notes: No (until the implementation is done)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.71%. Comparing base (24a50b4) to head (d1b3de3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4957   +/-   ##
=======================================
  Coverage   66.71%   66.71%           
=======================================
  Files         209      209           
  Lines       32052    32052           
=======================================
  Hits        21384    21384           
- Misses       9388     9390    +2     
+ Partials     1280     1278    -2     

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[mathetake]

feel like maybe calling it "weight" instead of usage maybe be better? idk

[mathetake]

hidden until envoyproxy/ratelimit#802

[mathetake]

hidden until envoyproxy/ratelimit#802

[mathetake]

will add the cel validation this afternoon (in japan)

[mathetake]

maybe simply Usage->HitAddends would be consistent

[mathetake]

@zirain @zhaohuabing ptal - I removed the implementation-hide as envoyproxy/ratelimit#802 got merged

[mathetake]

i will do the implementation in a subsequent PR

[zhaohuabing]

@zirain @zhaohuabing ptal - I removed the implementation-hide as envoyproxy/ratelimit#802 got merged

@mathetake the implementation comment is used to hide the unimplemented EG API in the EG docs, we normally hide the API at the API PR and unhide it at the implementation PR.

[zhaohuabing]

LGTM thanks!
Prefer to hide the API in the docs as it's not implemented yet.

[mathetake]

ah ok - will do

[mathetake]

@zhaohuabing hid the APIs!

[zirain]

LGTM, hold for a while as @arkodg may want to take a look.

[zhaohuabing]

LGTM defer to @arkodg

[mathetake]

👍