Return event fields by field name

Since a field may be present both in an event and its stream's scope,
this returns a list which contains all the fields by the name. The
examples are also changed to match this new API.

Signed-off-by: Xiaona Han <xiaonahappy13@163.com>
Acked-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>