function nrInjectHeaders({ agent }, cb) {
  const tx = agent.tracer.getTransaction()
  const headers = {}
  tx.insertDistributedTraceHeaders(headers)
  agent.headers = headers
  cb()
}
Inserts tracecontext into active transaction.
We put the headers on the agent to be used in assertions later.

@param {object} params to function
@param {object} params.agent agent instance
@param {function} cb function to run after setting inbound tracecontext