Add missing ABC for Metric (open-telemetry#391)

Fix open-telemetry#390

Author: GoPavel

opentelemetry-api/src/opentelemetry/metrics/__init__.py

@@ -87,13 +87,14 @@ class DefaultLabelSet(LabelSet):
     """
 
 
-class Metric:
+class Metric(abc.ABC):
     """Base class for various types of metrics.
 
     Metric class that inherit from this class are specialized with the type of
     handle that the metric holds.
     """
 
+    @abc.abstractmethod
     def get_handle(self, label_set: LabelSet) -> "object":
         """Gets a handle, used for repeated-use of metrics instruments.