/usr/lib/ruby/vendor_ruby/serverspec/matcher/be_monitored_by.rb is in ruby-serverspec 2.41.3-3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 | RSpec::Matchers.define :be_monitored_by do |monitor|
  match do |service|
    service.monitored_by?(monitor, @monitor_name)
  end
  description do 
    if @monitor_name
      "be monitored by #{monitor} with name #{@monitor_name}"
    else 
      "be monitored by #{monitor}" 
    end 
  end 
  chain :with_name do |name|
    @monitor_name = (name ? name : nil)
  end
end
 |