[Synthetics] add metric item aria label (#151939)

## Summary

This missing aria-label is causing console errors on the overview page.

x-pack/plugins/synthetics/public/apps/synthetics/components/monitors_page/overview/overview/metric_item_icon.tsx

@@ -117,7 +117,7 @@ export const MetricItemIcon = ({
                 }
               }}
             >
-              <EuiButtonIcon iconType="alert" color="danger" size="m" />
+              <EuiButtonIcon iconType="alert" color="danger" size="m" aria-label={ERROR_DETAILS} />
             </StyledIcon>
           }
           isOpen={configIdByLocation === isPopoverOpen}