Added "filter" field to filter containers by name

[EduJGURJC]

"filter" field has been added to filter containers by name with a regexp. There are two sub-fields:

• containers:
  • If this field is not set, metrics of All containers are sent.
  • If this field has a regexp, only the metrics of the containers whose names match the regexp are sent.
• exclude:
  • If this field is not set, no containers are excluded
  • If this field has a regexp, the containers whose names match the regexp are excluded.

Both fields can be combined to filter containers. For example, you can send metrics of all containers whose names starts with "example_" but exclude the container with name "example_2"

[JeroenKops]

Exactly what I need. Why isn't this pull request merged?