MOZ_TIMELINE and components

Hi, 
I was wondering whether one could make MOZ_TIMELINE work on a
per-component basis.

First of all, I'm not sure that building the timeline service only for
MOZ_TIMELINE builds is a good thing.
Though the macros should stay as is.

Then one could follow a scheme like that for debug or profile, to switch
MOZ_TIMELINE on only for a specific set of modules. So people looking at
timing information in their module could only switch it on there, and
don't waste runtime and logspace for all the other stuff.

I don't think that we'd need to go as far as having it a runtime option,
like it is for logging, though.

Thoughts?

Axel