Design an in-memory rolling event counter. The counter should support two operations: - `record(timestamp)`: record one event at the given timestamp, measured in seconds. - `count(timestamp)`: return the number of events that occurred in the last 300 seconds, including events whose timestamps are in the interval `[timestamp - 299, timestamp]`. Discuss and implement an approach for the single-machine case. Then discuss trade-offs around: - when to remove expired events, - whether to use a background cleanup thread, - how stale data can affect correctness, - memory usage under high traffic, and - concurrency concerns.