Metric, structured logging क्या है?

A metric is a named numeric measurement with labels, sampled or aggregated over time: a counter (messages received), a gauge (queue depth), a histogram (latency, as in Book 13, chapter 5). Structured logging writes each log event as a record of named fields — time, level, service, event, identifiers, values — rather than a sentence, so that logs can be queried like data.