fw/output: add label property to Metric

Add a "label" property to Metric that combines its name with its
classifiers into a single string.

doc/source/api/output.rst

@@ -497,6 +497,11 @@ A :class:`Metric` has the following attributes:
     or they may have been added by the workload to help distinguish between
     otherwise identical metrics.
 
+``label``
+    This is a string constructed from the name and classifiers, to provide a
+    more unique identifier, e.g. for grouping values across iterations. The
+    format is in the form ``name/cassifier1=value1/classifier2=value2/...``.
+
 
 :class:`Artifact`
 -----------------

wa/framework/output.py

@@ -602,6 +602,12 @@ class Metric(Podable):
         instance._pod_version = pod_version  # pylint: disable =protected-access
         return instance
 
+    @property
+    def label(self):
+        parts = ['{}={}'.format(n, v) for n, v in self.classifiers.items()]
+        parts.insert(0, self.name)
+        return '/'.join(parts)
+
     def __init__(self, name, value, units=None, lower_is_better=False,
                  classifiers=None):
         super(Metric, self).__init__()