feat: add lastSeenByEnvironment flag (#4393)

Closes:
[1-1193](https://linear.app/unleash/issue/1-1193/add-lastseenbyenvironment-flag)

---------

Signed-off-by: andreas-unleash <andreas@getunleash.ai>

src/lib/__snapshots__/create-config.test.ts.snap

@@ -81,6 +81,7 @@ exports[`should create default config 1`] = `
       "filterInvalidClientMetrics": false,
       "frontendNavigationUpdate": false,
       "googleAuthEnabled": false,
+      "lastSeenByEnvironment": false,
       "maintenanceMode": false,
       "messageBanner": {
         "enabled": false,
@@ -116,6 +117,7 @@ exports[`should create default config 1`] = `
       "filterInvalidClientMetrics": false,
       "frontendNavigationUpdate": false,
       "googleAuthEnabled": false,
+      "lastSeenByEnvironment": false,
       "maintenanceMode": false,
       "messageBanner": {
         "enabled": false,

src/lib/types/experimental.ts

@@ -27,7 +27,8 @@ export type IFlagKey =
     | 'emitPotentiallyStaleEvents'
     | 'configurableFeatureTypeLifetimes'
     | 'filterInvalidClientMetrics'
-    | 'frontendNavigationUpdate';
+    | 'frontendNavigationUpdate'
+    | 'lastSeenByEnvironment';
 
 export type IFlags = Partial<{ [key in IFlagKey]: boolean | Variant }>;
 
@@ -128,6 +129,10 @@ const flags: IFlags = {
         process.env.UNLEASH_NAVIGATION_UPDATE,
         false,
     ),
+    lastSeenByEnvironment: parseEnvVarBoolean(
+        process.env.LAST_SEEN_BY_ENVIRONMENT,
+        false,
+    ),
 };
 
 export const defaultExperimentalOptions: IExperimentalOptions = {

src/server-dev.ts

@@ -43,6 +43,7 @@ process.nextTick(async () => {
                         slackAppAddon: true,
                         configurableFeatureTypeLifetimes: true,
                         frontendNavigationUpdate: true,
+                        lastSeenByEnvironment: true,
                     },
                 },
                 authentication: {