Blueprint span goes missing when errors occur

[garthk]

I'm getting my root span from Opencensus.Plug.Trace and Opencensus.Absinthe.Middleware, but not the span from Opencensus.Absinthe.Phase.Pop.

[garthk]

Dug into it as part of #17. Absinthe just crashes straight out. I reckon I'll need special handling in Opencensus.Absinthe.Plug to:

• Hook execution after the plugs finish e.g. Plug.Exception, Opencensus.Plug.Trace
• Spot ocp thinking it's deeper-nested than plausible
• Whileocp thinks we're in the right trace but not the right span:
  • If it's one of our spans, add attributes indicating a crash-out
  • ocp.finish_span/0

[garthk]

Defining my own call/2 and wrapping the call to super/2 will give me a crack at it.