Use fiber-profiler gem.

gems.rb

@@ -10,6 +10,7 @@
 gemspec
 
 # gem "io-event", git: "https://github.com/socketry/io-event.git"
+gem "fiber-profiler"
 
 group :maintenance, optional: true do
 	gem "bake-gem"

lib/async/scheduler.rb

@@ -15,6 +15,14 @@
 require "resolv"
 
 module Async
+	begin
+		require "fiber/profiler"
+		Profiler = Fiber::Profiler
+	rescue LoadError
+		# Fiber::Profiler is not available.
+		Profiler = nil
+	end
+
 	# Handles scheduling of fibers. Implements the fiber scheduler interface.
 	class Scheduler < Node
 		DEFAULT_WORKER_POOL = ENV.fetch("ASYNC_SCHEDULER_DEFAULT_WORKER_POOL", nil).then do |value|
@@ -42,7 +50,7 @@ def self.supported?
 		# @public Since *Async v1*.
 		# @parameter parent [Node | Nil] The parent node to use for task hierarchy.
 		# @parameter selector [IO::Event::Selector] The selector to use for event handling.
-		def initialize(parent = nil, selector: nil, profiler: IO::Event::Profiler.default, worker_pool: DEFAULT_WORKER_POOL)
+		def initialize(parent = nil, selector: nil, profiler: Profiler&.default, worker_pool: DEFAULT_WORKER_POOL)
 			super(parent)
 
 			@selector = selector || ::IO::Event::Selector.new(Fiber.current)