Add database link to span context

[felixbarny]

Continuation of #676: add the db link to the span context

apm server issue: elastic/apm-server#2478

@wolframhaussig are you interested in implementing this?

[wolframhaussig]

@felixbarny I can try. But currently, the discussion in elastic/apm#107 is not yet completed so I would want to wait for that. And can you confirm what I should implement exactly as the field has to be transferred to the APM server and I am not sure if you would want to implement that youself. Just so I do not implement more than required...

[felixbarny]

We've agreed to add the field and there's already an APM Server issue for that: elastic/apm-server#2478.

Even if there's no conclusion yet about how to parse MSSql DBLinks, we can go ahead with what we currently have.

You'd have to add a dbLink property to co.elastic.apm.agent.impl.transaction.Db, reset it to null in Db#resetState and serialize it in DslJsonSerializer.

The current APM Server would just ignore the field but once elastic/apm-server#2478 is implemented and released, the field is available in ES.