consider swagger 3 upgrade

[codefromthecrypt]

From #46

The spans POST documentation is still a bit misleading. With the parameter named "spans", it seems to imply that it's expecting

{
    "spans": [{<...span...>}, {<....span...>}]
}

but it actually wants:

[{<...span...>}, {<...span...>}]

I couldn't figure out a way to acurately represent this in Swagger 2. It might be possible using Swagger 3 request body descriptions.