Document support of protobuf extension

[benjamin-cooper-ai]

From: #11

We've since landed extension support in Protozero, a way to use custom extension fields inside TrackEvents and support for bundling proto extension descriptors together with the trace so that Trace Processor and Perfetto UI can make sense of the custom types.

So, in theory all the pieces are available, but perhaps the most important part is still work in progress: documentation. Stay tuned!

Can this feature please be publicly documented? I don't feel comfortable using an undocumented feature and it'd be nice to get this feature over the last mile.

[LalitMaganti]

Ack thanks for calling this out. We have a big docs refactor planned for Q4, I'll try to take a look at this when that happens.

[LalitMaganti]

Just to be clear, this feature is relied upon heavily by Chrome so you can be rest assured even if it's underdocumented, it's not going anywhere and you can rely on it :)