Allow defining default_tags only for rails group

[magec]

Currently, yabeda-rails plugin do not have into account :rails group default_labels when populating metrics. This change allows defining tags for :rails group, so if you are using other yabeda plugins they don't get populated with them.

The main problem here is that we are using yabeda-rails and yabeda-puma in the same application. If we set default_tags, both rails and puma metrics will have that tag populated. If we use the :group option setting it to rails (so the tag is only added to rails metrics), the plugin ignores it. This change makes rails plugin aware of :rails groups metrics.

[magec]

Is there any chance this could get merged @Envek ? asking for a friend