Design Multi-Dimensional Request Rate Limiting

Design a rate limiter for a backend service. Part 1: Build a standard rate limiter that can limit incoming requests by a key such as user ID, IP address, API key, or endpoint. The limiter should work in a distributed service environment with multiple application servers. Part 2: Extend the design so that each request may contain several fields, and different rate-limit rules may apply to different fields. For example, a request might include a user ID, an IP address, an endpoint, a region, and an operation type, and the system may need to enforce separate limits for each field or combination of fields. Discuss APIs, data model, algorithms, consistency, scalability, failure handling, and how the request should be accepted or rejected when multiple limits apply.