Pass rangeOfInterest to metrics for the metrics side panel

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 2906d5c8ee13ba624bf3928f7767217c5ff47c5d..d0f52ef71fca99040d457c4e7f9c726cd2bb3bfd 100644
--- a/tracing/tracing/metrics/metric_registry.html
+++ b/tracing/tracing/metrics/metric_registry.html
@@ -24,8 +24,10 @@ tr.exportTo('tr.metrics', function() {
     if (!(metric instanceof Function))
       throw new Error('Metrics must be functions');
 
-    if (metric.length !== 2)
-      throw new Error('Metrics take a ValueSet and a Model');
+    if (metric.length < 2) {
+      throw new Error('Metrics take a ValueSet and a Model and ' +
+                      'optionally an options dictionary');
+    }
   });
 
   return {