The question is about tracking and storing individual events (logs) with arbitrary per-item detail vs. dimensionally limited aggregations (time series / metrics). In either case, I think we agree that the data should be recorded in a structured way, and when I say "logs" I just mean a record of individual items, not of sampled/aggregated metrics.
Given that, you need both logs (individual events) and metrics. Logs give you crucial insight into individual interesting events such as single requests that bring your service down, but logs are orders of magnitude more expensive than metrics in tracking, storage, and processing. So that's why you use metrics for a much wider scope and for longer time periods.
