Demo script to show that the effect of tracing an instance method is global across all instances of that class.

affect-all-instances.rb

class Foo
  def hello(arg = nil)
    puts "dale sent me"
  end
end

one = Foo.new
two = Foo.new

one.hello
two.hello

# so this is a weird one, the call event happens right after
# entering a method instead of before a call happens, as one
# might intuitively expect
trace = TracePoint.new(:call) { |tp| puts "intercepted! arg=#{tp.binding.local_variable_get(:arg)}" }
trace.enable(target: Foo.instance_method(:hello)) do
  one.hello(:first)
  two.hello(:second)
end

output.txt

[tmp]# ruby -v affect-all-instances.rb 
ruby 2.6.3p62 (2019-04-16 revision 67580) [x86_64-linux]
dale sent me
dale sent me
intercepted! arg=first
dale sent me
intercepted! arg=second
dale sent me