Can't mass-assign protected attributes: ignore_unknown_metrics on executing rake vizir:init

I was getting the "mass-assign protected attributes: ignore_unknown_metrics" error while running rake vizir:init

The issue was occurring in the following line:
p.assign_attributes(attrs)

Rake file name: vizir.rake

Added :ignore_unknown_metrics to attr_custom in CollectdProvider class.

This was the error, I was facing on runing rake vizir:init:
  
saurav@saurav-Vostro-1540:~/workspace/vizir$ rake vizir:init --trace
** Invoke vizir:init (first_time)
** Invoke vizir:setup (first_time)
** Invoke environment (first_time)
** Execute environment
** Execute vizir:setup
** Execute vizir:init
rake aborted!
Can't mass-assign protected attributes: ignore_unknown_metrics
/home/saurav/.rvm/gems/ruby-1.9.3-p125@rails/gems/activemodel-3.2.12/lib/active_model/mass_assignment_security/sanitizer.rb:48:in `process_removed_attributes'
/home/saurav/.rvm/gems/ruby-1.9.3-p125@rails/gems/activemodel-3.2.12/lib/active_model/mass_assignment_security/sanitizer.rb:20:in `debug_protected_attribute_removal'