[APM] Synthtrace: Add support for log events

[sorenlouv]

Context

In elastic/apm-server#6544 @stuartnelson3 was trying to get log events to show up in APM (under the Logs tab). He was initially seeing some problems that could indicate that log correlation wasn't working as it should. Even though the log events eventually showed up in APM, this reminded me that we don't have any automated tests (api or e2e) for this feature.

Suggestion

Synthtrace is right not solely focused on generating APM events. I think this makes sense and Synthtrace should always be an APM-centric data generator. However, I think we should consider adding support for generating non-APM events also.
In this case I think Synthtrace should be able to generate the simplest form of log events that will then be displayed in APM.

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[yngrdyn]

There are some mobile fields that right now could be extracted from logs, for example mobile events where we could get information such as app_launch_count

[sorenlouv]

Replaced by #170133