metrics: load plugins from config file options

Only use the default plugins if there are no plugins listed in the
config.

The list of all plugins and the default list is also now stored
in metrics.py, along with the plugins definitions
metrics: plugin: add standard constructor

Allows swallowing of arguments to avoid calling object's contructor
with parameters, leading to exception
metrics: add standard 'threshold' parameter

For those plugins that won't define one
metrics: add process state plugin

Checks if a process is running