Rename ValueSet* to HistogramSet*.

tracing/tracing/metrics/metric_registry.html

Index: tracing/tracing/metrics/metric_registry.html
diff --git a/tracing/tracing/metrics/metric_registry.html b/tracing/tracing/metrics/metric_registry.html
index d0f52ef71fca99040d457c4e7f9c726cd2bb3bfd..42d91acb802121d1d9186028f9d1a9151248c0a4 100644
--- a/tracing/tracing/metrics/metric_registry.html
+++ b/tracing/tracing/metrics/metric_registry.html
@@ -25,7 +25,7 @@ tr.exportTo('tr.metrics', function() {
       throw new Error('Metrics must be functions');
 
     if (metric.length < 2) {
-      throw new Error('Metrics take a ValueSet and a Model and ' +
+      throw new Error('Metrics take a HistogramSet and a Model and ' +
                       'optionally an options dictionary');
     }
   });