Add support for merging traces

[LalitMaganti]

It's become clear that we need some way to have multiple independent traces (collected on either the same device or different devices) visualized and analyzable on a single timeline.

Most of the work for this already exists in the trace processor but it is very hard for anyone who does not understand a lot of the internals of TP to make use of it.

This bug tracks the remaining implementation which involves some small work in TP and then some non-trivial work in the UI to expose this functionality.

[LalitMaganti]

Will be looking to this actively in the coming month.

[LalitMaganti]

Current state:

1. I'm now pretty happy with the UI part of things, we have a pretty initial draft for "clock synchronisation" between traces.
2. There is some work to be done in trace processor to take the result of what the UI builds for clock sync and actually make use of this while parsing. This will require defining the JSON schema we expect folks to use and making the UI write it as well
3. Work on multiple machines has not started yet but I remain convinced it's not too much work on top of what we already are doing for single machine merging.

[sifangli-labs]

@LalitMaganti Thanks for woring on this impritant feature! If the user does not care about multiple machines and the UI function, is there a cmdline util that works to merge two pftraces from the same machine (captured around the same period) into a single pftrace file?
If so any instructions?
Thanks!

[LalitMaganti]

No sorry you will have to wait until this is done. I'm not giving preview instructions, this is not yet complete.

From my previous comment

There is some work to be done in trace processor to take the result of what the UI builds for clock sync and actually make use of this while parsing. This will require defining the JSON schema we expect folks to use and making the UI write it as well