ContextWithSpan returns a new `context.Context` that holds a reference to the provided `span`.

SpanFromContext returns the `Span` previously associated with `ctx`, or a no-op span if none is found.

StartRootSpan starts a new top-level span and registers it with the given context.

StartSlowSpan starts a new top-level slow span and registers it with the given context.

Span represents an operation that is named and timed.