Add error handling on page.on(metric) handler

The most important thing to do is to handle the error and ensure that
the page.on metric returns false so that the metric tag isn't update.
This is being defensive, since the context has already closed and the
metric emitter will not emit a metric when the context has closed.

We're logging a debug log here since this information will not benefit
the end user and when this error occurs they can't action on it. It
could be useful for debugging though.

common/page.go

@@ -485,17 +485,21 @@ func (p *Page) urlTagName(url string, method string) (string, bool) {
 	p.eventHandlersMu.RLock()
 	defer p.eventHandlersMu.RUnlock()
 	for _, h := range p.eventHandlers[EventPageMetricCalled] {
-		func() {
+		err := func() error {
 			// Handlers can register other handlers, so we need to
 			// unlock the mutex before calling the next handler.
 			p.eventHandlersMu.RUnlock()
 			defer p.eventHandlersMu.RLock()
 
 			// Call and wait for the handler to complete.
-			h(PageOnEvent{
+			return h(PageOnEvent{
 				Metric: em,
 			})
 		}()
+		if err != nil {
+			p.logger.Debugf("urlTagName", "handler returned an error: %v", err)
+			return "", false
+		}
 	}
 
 	// If a match was found then the name field in em will have been updated.