Abstract MetricsCollector class

This will force people to implement this methods of this class when they
refer to it.

luigi/metrics.py

@@ -1,3 +1,5 @@
+import abc
+
 from enum import Enum
 
 
@@ -17,21 +19,27 @@ def get(cls, which):
             raise ValueError("MetricsCollectors value ' {0} ' isn't supported", which)
 
 
-class MetricsCollector(object):
+class MetricsCollector(abc.ABC):
     """Dummy MetricsCollecter base class that can be replace by tool specific
     implementation.
     """
+
+    @abc.abstractmethod
     def __init__(self):
         pass
 
+    @abc.abstractmethod
     def handle_task_started(self, task):
         pass
 
+    @abc.abstractmethod
     def handle_task_failed(self, task):
         pass
 
+    @abc.abstractmethod
     def handle_task_disabled(self, task, config):
         pass
 
+    @abc.abstractmethod
     def handle_task_done(self, task):
         pass