Register metric Histogram names.

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 7538d743805c544092efd2b92c62bd1c5755ad7a..ced5222b04a348b772338670c2db1d3f0e358744 100644
--- a/tracing/tracing/metrics/metric_registry.html
+++ b/tracing/tracing/metrics/metric_registry.html
@@ -20,13 +20,31 @@ tr.exportTo('tr.metrics', function() {
 
   MetricRegistry.addEventListener('will-register', function(e) {
     var metric = e.typeInfo.constructor;
-    if (!(metric instanceof Function))
+    var metadata = e.typeInfo.metadata;
+
+    if (!(metric instanceof Function)) {
       throw new Error('Metrics must be functions');
+    }
 
     if (metric.length < 2) {
       throw new Error('Metrics take a HistogramSet and a Model and ' +
                       'optionally an options dictionary');
     }
+
+    if (!(metadata.histogramNames instanceof Set)) {
+      throw new Error('Metrics must register the set of Histogram names ' +
+          'that they may produce');
+    }
+
+    for (var typeInfo of MetricRegistry.getAllRegisteredTypeInfos()) {
+      for (var name of metadata.histogramNames) {
+        if (typeInfo.metadata.histogramNames.has(name)) {
+          throw new Error('Multiple metrics cannot produce Histograms ' +
+              'with the same name: ' + name + ' ' + metric.name + ' ' +
+              typeInfo.constructor.name);
+        }
+      }
+    }
   });
 
   return {