fix(telemetry): measure link/button clicks properly (#10707)

We assumed that the click happens on the element with data-glean,
but this is not the case if the click happens on a child of the link/button.

client/src/telemetry/glean-context.tsx

@@ -172,14 +172,14 @@ const gleanAnalytics = glean();
 const GleanContext = React.createContext(gleanAnalytics);
 
 function handleButtonClick(ev: MouseEvent, click: (source: string) => void) {
-  const button = ev?.target;
+  const button = (ev?.target as HTMLElement | null)?.closest("button");
   if (button instanceof HTMLButtonElement && button.dataset.glean) {
     click(button.dataset.glean);
   }
 }
 
 function handleLinkClick(ev: MouseEvent, click: (source: string) => void) {
-  const anchor = ev?.target;
+  const anchor = (ev?.target as HTMLElement | null)?.closest("a");
   if (anchor instanceof HTMLAnchorElement) {
     if (anchor.dataset.glean) {
       click(anchor.dataset.glean);