Nguyen Trong Nghia

[Feature Request] Instrument Metrics for Errors Returned by ShouldRateLimit gRPC Calls

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Instrument metrics for errors returned by ShouldRateLimit gRPC calls so that we can monitor those failures and set up alerts. Can I submit a PR?

Bump mediocregopher/radix to v4

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### Problem The `ratelimit` package currently depends on [`mediocregopher/radix` v3](https://github.com/mediocregopher/radix/tree/v3), which does not support context-aware operations. As a result, when a context is canceled (e.g., due to client timeouts), `ratelimit`...