[META 605] Add span links for message tracing

[![issue details](https://img.shields.io/endpoint?label=meta-issue:&url=https%3A%2F%2Fgiss.app.elstc.co%2Fapi%2Fstatus%2Felastic%2Fapm%2F605)](https://github.com/elastic/apm/issues/605)

[![issue details](https://img.shields.io/endpoint?label=spec-issue:&url=https%3A%2F%2Fgiss.app.elstc.co%2Fapi%2Fstatus%2Felastic%2Fapm%2F606)](https://github.com/elastic/apm/issues/606)

