Yeah, I think that’s good advice for client apps. I didn’t know Span defined a behavior and hadn’t thought about mocking the set_attributes method, so that’s good to know!

I’m actually working on a library that defines and links spans so it’s nice to have a bit more control. A couple folks on slack pointed me toward the pid and ets exporters. The example they linked registers the pid exporter to send spans to the test process and then assert that the received span has the correct attributes.

setup do
    :application.stop(:opentelemetry)
    :application.set_env(:opentelemetry, :tracer, :ot_tracer_default)

    :application.set_env(:opentelemetry, :processors, [
      {:ot_batch_processor, %{scheduled_delay_ms: 1}}
    ])

    :application.start(:opentelemetry)

    :ot_batch_processor.set_exporter(:ot_exporter_pid, self())
    :ok
  end

  test "records spans for Phoenix web requests" do
    OpentelemetryPhoenix.setup()

    :telemetry.execute(
      [:phoenix, :endpoint, :start],
      %{system_time: System.system_time()},
      %{conn: @conn, options: []}
    )

    :telemetry.execute(
      [:phoenix, :router_dispatch, :start],
      %{system_time: System.system_time()},
      %{
        conn: @conn,
        plug: MyStoreWeb.UserOrdersController,
        plug_opts: :index,
        route: "/users/{user_id}/orders",
        path_params: %{"user_id" => "123"}
      }
    )

    :telemetry.execute(
      [:phoenix, :endpoint, :stop],
      %{duration: 444},
      %{conn: stop_conn(@conn), options: []}
    )

    expected_status = OpenTelemetry.status(:Ok, "Ok")

    assert_receive {:span,
                    span(
                      name: "GET /users/{user_id}/orders",
                      attributes: list,
                      status: ^expected_status
                    )}

    assert [
             {"http.client_ip", "10.211.55.2"},
             {"http.host", "mystore"},
             {"http.method", "GET"},
             {"http.scheme", "http"},
             {"http.status", 200},
             {"http.target", "/users/123/orders"},
             {"http.user_agent",
              "Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:79.0) Gecko/20100101 Firefox/79.0"},
             {"net.host.ip", "10.211.55.2"},
             {"net.host.port", 4000},
             {"net.peer.ip", "10.211.55.2"},
             {"net.peer.port", 64291},
             {"net.transport", "IP.TCP"},
             {"phoenix.action", "index"},
             {"phoenix.plug", "Elixir.MyStoreWeb.UserOrdersController"}
           ] == List.keysort(list, 0)
  end