Make default hitsAddend minimum value configurable

[gcalmettes]

fix: #730

In the current implementation of the ratelimit service, the hitsAddend value that is associated with each request is set to a minimum of 1.
This is done to ensure that if the RateLimit request does not specify any request.hitsAddend (like what the envoy ratelimit filter does by default if no envoy.ratelimit.hits_addend has been defined), the hit is still counted.

This minimum hitsAddend of 1 is enforced via a Max(1, <hitsAddend from request>) function.

This PR adds the possibility to configure the minimum value of the hitsAddend via a settings, so 0 can also be used as minimum hitsAddend value if needed. The current behavior of 1 being the default value is preserved, and if the environment variable is not set, then a value of 1 is applied, like currently.

The ability to configure a minimum hitsAddend to zero can help for example for the following use cases:

• having the possibility to send a RateLimitRequest to check the current remaining rates without consuming a quota (e.g.: Add a way to check rate limits without using quota #244)
• supporting the cases when the "weight" of the request is not known during the request but on the response (e.g.: rate limiting llm tokens as per Send ratelimit request to ratelimit service on response flow with hits_addend envoy#29161 (comment)). In this case, we still want to ratelimit on the request, but we do not want the request itself to add an additional arbitrary 1 value to the real weight defined as hitsAddend.

[gcalmettes]

@envoyproxy/ratelimit-maintainers

@zirain I tried to ping @envoyproxy/ratelimit-maintainers like described in the CONTRIBUTING.md document but it seems that the handle is not available. Any chance you would know who I could ping for this PR ? Thanks a lot.

[zirain]

I think @mattklein123 is the person, hope he have some free slot to review.

[gcalmettes]

Any chance you could take a look @mattklein123 ? Thanks a lot.