forces a re-prompt of telemetry opt-in when Kibana version changes

resolves elastic#49519

If a user has previously opted out of telemetry, we are making a
change to re-prompt them when the version of Kibana they are
running changes.  Previously, once opted out, they would never
get prompted again.

src/legacy/core_plugins/telemetry/index.ts

@@ -48,6 +48,9 @@ const telemetry = (kibana: any) => {
         // `config` is used internally and not intended to be set
         config: Joi.string().default(Joi.ref('$defaultConfigPath')),
         banner: Joi.boolean().default(true),
+        lastVersionChecked: Joi.string()
+          .allow('')
+          .default(''),
         url: Joi.when('$dev', {
           is: true,
           then: Joi.string().default(

src/legacy/core_plugins/telemetry/mappings.json

@@ -3,6 +3,9 @@
     "properties": {
       "enabled": {
         "type": "boolean"
+      },
+      "lastVersionChecked": {
+        "type": "keyword"
       }
     }
   }

src/legacy/core_plugins/telemetry/server/get_telemetry_opt_in.ts

@@ -17,7 +17,7 @@
  * under the License.
  */
 
-export async function getTelemetryOptIn(request: any) {
+export async function getTelemetryOptIn(request: any): Promise<boolean | null> {
   const isRequestingApplication = request.path.startsWith('/app');
 
   // Prevent interstitial screens (such as the space selector) from prompting for telemetry
@@ -27,9 +27,9 @@ export async function getTelemetryOptIn(request: any) {
 
   const savedObjectsClient = request.getSavedObjectsClient();
 
+  let savedObject;
   try {
-    const { attributes } = await savedObjectsClient.get('telemetry', 'telemetry');
-    return attributes.enabled;
+    savedObject = await savedObjectsClient.get('telemetry', 'telemetry');
   } catch (error) {
     if (savedObjectsClient.errors.isNotFoundError(error)) {
       return null;
@@ -43,4 +43,16 @@ export async function getTelemetryOptIn(request: any) {
 
     throw error;
   }
+
+  const { attributes } = savedObject;
+  if (attributes.enabled !== false) return attributes.enabled;
+
+  // Additional check if they've already opted out (enabled: false) - if the
+  // Kibana version has changed since they opted out, set them back to the null
+  // state to get prompted again.
+  const config = request.server.config();
+  const kibanaVersion = config.get('pkg.version');
+  if (kibanaVersion !== attributes.lastVersionChecked) return null;
+
+  return attributes.enabled;
 }

src/legacy/core_plugins/telemetry/server/routes/opt_in.ts

@@ -36,11 +36,13 @@ export function registerOptInRoutes(core: CoreSetup) {
     },
     handler: async (req: any, h: any) => {
       const savedObjectsClient = req.getSavedObjectsClient();
+      const lastVersionChecked = server.config().get('pkg.version');
       try {
         await savedObjectsClient.create(
           'telemetry',
           {
             enabled: req.payload.enabled,
+            lastVersionChecked,
           },
           {
             id: 'telemetry',