Annotation header with descriptor types

It would be helpful to parsers for the first line of metrics descriptors to contain the descriptor type and version. Moving this discussion from an email thread...

Speaking of versioning and such, I would really appreciate it if all of these documents came with a header saying what the document type and version is. At present I need to guess based on the filename and first line of content which is both hacky and will break when new versions roll out. Actually, to differentiate between bridge and relay descriptors I need to rely on the '10.x.x.x' scrubbing... :/

Thoughts on how we should format it?

Cheers! -Damian