feat: Add callback option to config to capture context when starting transactions and spans

[cjr125]

No description provided.

[cla-checker-service]

💚 CLA has been signed

[vigneshshanmugam]

Not sure whats the use-case we are solving with this. Why not use Global Labels for this functionality - https://www.elastic.co/guide/en/apm/agent/rum-js/current/agent-api.html#apm-add-labels

[cjr125]

Not sure whats the use-case we are solving with this. Why not use Global Labels for this functionality - https://www.elastic.co/guide/en/apm/agent/rum-js/current/agent-api.html#apm-add-labels

The specific use case for the callback is to capture context specific data which is different between the call stacks when firing / handling the transaction / span events from the auto-instrumentation. For example, when an application uses a combination of javascript files in which some import the RUM agent and others do not, there will be different stack traces when comparing the stack frame where the event was fired as opposed to handled. The ultimate goal would still be to assign labels, the gap being the context when the event is fired in cases where that context does not import the RUM agent library

[cjr125]

@vigneshshanmugam any update on this? do you understand the use case?

[malek-andrew]

@vigneshshanmugam Customer filed a support case today regarding this PR. Would you mind give it another review?