Your question is Design a Prometheus-Style Metrics System. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
Design a counter/metrics system like Prometheus/Datadog counters: clients (mobile and web) send many kinds of events through an SDK to the backend; mobile clients may send large batches after being offline; no event may be lost; aggregate events for dashboards with a data-freshness requirement of tens of minutes.
Asked in the system design (60 min) stage. Candidate described the prompt in Chinese; translated.
Constraints include no silent event loss, delayed mobile batches, tens-of-minutes freshness, bounded operating cost, and safe handling of duplicate or malformed events.