A trace is a collection of spans, which are objects that represent the work being done by individual services or components involved in a request as it flows through a system. A span contains a span context, which is a set of globally unique identifiers that represent the unique request that each span is a part of. Your system may handle dozens, hundreds, thousands, or millions of requests per second – each of these requests would have a single trace identifier, and each span in that request would have a unique span identifier. This span context is immutable and cannot be modified after creation.
A metric is some raw measurement about a service, captured at runtime. Logically, the moment of capturing one of these measurements is known as a metric event which consists not only of the measurement itself, but the time that it was captured. These raw measurements are then used by monitoring and alerting systems to provide statistical data about the performance of a service or system.