<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def span_path(cls, project, trace, span):
 """Return a fully-qualified span string."""

        return google.api_core.path_template.expand(
            "projects/{project}/traces/{trace}/spans/{span}",
            project=project,
            trace=trace,
            span=span,
        )