[Schema v2] Migrate HTTP conventions

[joaopgrassi]

Migrate the HTTP conventions to the schema v2 as a prototype.

[Dipanshusinghh]

Hey @joaopgrassi, can I take this one?

I went through the v2 docs and the existing HTTP convention
files - looks like we need to rewrite them to use the new
definition/2 format with proper spans, metrics, attribute
groups and entity_associations.

Already have a rough draft ready. Can you assign this to me?