feat(ddm): Update onboarding button copy (#64251)

- closes https://github.com/getsentry/sentry/issues/64038

static/app/views/ddm/pageHeaderActions.tsx

@@ -138,7 +138,7 @@ export function PageHeaderActions({showCustomMetricButton, addCustomMetric}: Pro
     <ButtonBar gap={1}>
       {showCustomMetricButton && (
         <Button priority="primary" onClick={() => addCustomMetric('header')} size="sm">
-          {t('Add Custom Metric')}
+          {t('Set Up Custom Metrics')}
         </Button>
       )}
       <Button