Plumb trace canonicalUrl through TelemetryInfo.

tracing/tracing/metrics/metric_map_function.html

Index: tracing/tracing/metrics/metric_map_function.html
diff --git a/tracing/tracing/metrics/metric_map_function.html b/tracing/tracing/metrics/metric_map_function.html
index 8ba8196edd01e6f630130f37c4071e82114b4aae..4f0ed8441a7bb03650c3a30414575c0f0b830de3 100644
--- a/tracing/tracing/metrics/metric_map_function.html
+++ b/tracing/tracing/metrics/metric_map_function.html
@@ -70,6 +70,13 @@ tr.exportTo('tr.metrics', function() {
     for (const metadata of model.metadata) {
       if (!metadata.value || !metadata.value.telemetry) continue;
 
+      const traceUrls = metadata.value.telemetry[
+        tr.v.d.RESERVED_NAMES.TRACE_URLS];
+      if (traceUrls && model.canonicalUrl !== traceUrls[0]) {
+        throw new Error(`canonicalUrl "${model.canonicalUrl}" != ` +
+            `traceUrl "${traceUrls[0]}"`);
+      }
+
       for (const [name, value] of Object.entries(metadata.value.telemetry)) {
         const type = tr.v.d.RESERVED_NAMES_TO_TYPES.get(name);
         if (type === undefined) {