Spans are orphaned when using datadog backend

[clarkenciel]

Hi! First, thanks for providing these tracing facades so we don't have to monkeypatch async.

I currently use datadog for APM and have observed that APM spans get orphaned in code running inside of tasks spawned by async. Going by this issue it seems to be because datadog doesn't support fiber-based context. I've seen the issues you've created and found my way to the traces family of gem.

When I try to use Traces with the datadog backend I still see orphaned spans so I wonder if I'm using it incorrectly. For example, I've got the following test script which I run with TRACES_BACKEND=traces/backend/datadog ruby async.rb:

# frozen_string_literal: true

require "bundler/inline"

gemfile do
  source "https://rubygems.org"

  gem "async"
  gem "traces"
  gem "traces-backend-datadog"
  gem "datadog"
end

# Now you can use these gems in your script
require "datadog"
require "traces"
require "async"
require "async/barrier"
require "async/semaphore"

Datadog.configure do |c|
  c.service = "async-traces-test"
  c.env = "development"
end

count = 5
100.times do # 100 times to make sure they show up despite datadog's trace sampling
  Traces.trace("async-traces-test", attributes: {version: 2}) do
    Sync do
      barrier = Async::Barrier.new
      semaphore = Async::Semaphore.new(10, parent: barrier)

      Traces.trace("start tasks", attributes: {count:})
      Array.new(count) do
        semaphore.async do
          sleeptime = rand(0.1..0.9)
          Traces.trace("task", attributes: {sleeptime:}) do
            sleep(sleeptime)

            # the "traces" library/interface doesn't have something like `set_tag`
            # so we reach into datadog directly
            Datadog::Tracing.active_span.set_tag("result", sleeptime * 1000)
          end
        end
      end.map(&:wait)
    end
  end
end

And this produces traces like so:

Note that while all the spans make their way into APM, the child traces are detached from their parents. My hope was that these libraries would manage these links automatically.