Support propagating OpenTelemetry context

[rewritten]

This change adds optional support for propagating OpenTelemetry contexts in tasks spawned by the Async and Batch middleware.

This allows to correlate spans in otel backends (like datadog and others), so something like

field :posts, :list_of(:post) do
  resolve fn user, _, _ -> async(fn -> PostsResolver.posts_by_user(user) end) end
end

defmodule PostsResolver do
  def posts_by_user(user) do
    OpenTelemetry.Tracer.with_span("data access: posts by user") do
      # query posts
    end
  end
end

will create a child span in the existing trace, instead of creating a totally unrelated span.

No change in client code is necessary, and users that already propagate the context manually will just have one more indirection.

[kdawgwilk]

This is fantastic! 👏

[kdawgwilk]

Does https://github.com/spandex-project/spandex have a similar Task you can delegate to?

EDIT: created issue over there spandex-project/spandex#148

[benwilson512]

Happy to merge this, thanks!