Expand Result to Include Signal Types

[rubvs]

Related to solving https://github.com/elastic/apm-managed-service/issues/606 for incoming APM events (not needed for OTEL).

Context

We need to encode the incoming signal type into the Result datastructure, which we can then use within ingest service and index service to extend the necessary metric attributes.

Fundamentally, Result is an abstraction that encodes information after we have processed a set of events, called a batch. It operates on the level of a batch not an event.

Therefore, we don't have information on the event level. We need to update Result to encode signal types. There are 3 ways of doing this:

1. Update the Accepted, and related, fields to a map datastructure
2. Create a Signal struct, or something similar, to containerize data
3. Manually add the different fields

Solution

Updating the current fields (1) will introduce breaking changes to both APM Server and MIS, since Result operates on the protocol level. Creating a new struct (2) is redundant, but I"m open to suggestions. Therefore, manually adding fields (3) is the simplest change.

[marclop]

Thanks!