function injected(agent) {
  const { headers } = agent
  const { traceparent } = headers
  const fields = traceparent.split('-')
  return {
    traceId: fields[1],
    spanId: fields[2],
    sampled: fields[3] === '01'
  }
}
Returns span context from injected traceparent header

@param {object} agent instance
@returns {object} span context from injected traceparent header