Prometheus: Fix running of health check query based on access mode (#42189)

* Prometheus: Use this.query for health check

* Update alertMessage in e2e tests

e2e/various-suite/specs/exemplars.spec.ts

@@ -4,7 +4,7 @@ const dataSourceName = 'PromExemplar';
 const addDataSource = () => {
   e2e.flows.addDataSource({
     type: 'Prometheus',
-    expectedAlertMessage: 'Bad Gateway',
+    expectedAlertMessage: 'Error reading Prometheus',
     name: dataSourceName,
     form: () => {
       e2e.components.DataSource.Prometheus.configPage.exemplarsAddButton().click();