Design a Rolling Event Counter

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.