Add unit multiplier

[harpunius]

Implements the unit multiplier idea suggested in #190.

rate_limit:
  unit: <second, minute, hour, day>
  requests_per_unit: <uint>
  unit_multiplier: <uint | optional>

• feat: add rate limit unit multiplier
• feat(xds): add rate limit unit multiplier

I chose to force unit multiplier to 1 if the value is unset to avoid if/else checks whenever we rely on the value or log anything. This means the PR got slightly bigger than intended, but I think it's nicer.

The config parser panics if unit_multiplier: 0 is set. One RFC is the TODO in UnitToDividerWithMultiplier where I added a redundant if-zero runtime check to make sure we don't multiply by zero and effectively disable the rate limiting.

Note, this is a draft and doesn't build due to the dependencies on both https://github.com/envoyproxy/go-control-plane/blob/main/envoy/service/ratelimit/v3/rls.pb.go#L357-L367 and https://github.com/envoyproxy/go-control-plane/blob/main/ratelimit/config/ratelimit/v3/rls_conf.pb.go#L246-L263. I opened an issue to track this: envoyproxy/envoy#33277.

I couldn't run the integration tests locally (WSL setup) so let me know if we need to extend those tests as well.

Happy to hear your thoughts.

[github-actions]

This pull request has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in 7 days if no further activity occurs. Please feel free to give a status update now, ping for review, or re-open when it's ready. Thank you for your contributions!

[harpunius]

Not stale. Still waiting for input from e.g. @mattklein123