[Fleet] Cap setup attempts to 50 on Serverless (#171550)

## Summary

If there is a bug in Fleet setup, we can retrigger rollovers on each
attempt, causing shard explosion in Elasticsearch. We need a sane limit
to prevent this from happening, which this PR introduces.

The impact of Fleet setup failing to complete (Agents may not be able to
be enrolled) is much smaller than causing shard explosion, so this seems
like an acceptable tradeoff.

This is only a small part of the overall solution - in the current
incident we have, it's still unclear why we are failing to rollover the
index and getting into this loop.

### Checklist

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x-pack/plugins/fleet/server/plugin.ts

@@ -524,6 +524,9 @@ export class FleetPlugin
 
     this.policyWatcher.start(licenseService);
 
+    // We only retry when this feature flag is enabled (Serverless)
+    const setupAttempts = this.configInitialValue.internal?.retrySetupOnBoot ? 25 : 1;
+
     const fleetSetupPromise = (async () => {
       try {
         // Fleet remains `available` during setup as to excessively delay Kibana's boot process.
@@ -555,10 +558,9 @@ export class FleetPlugin
             );
           },
           {
-            // We only retry when this feature flag is enabled
-            numOfAttempts: this.configInitialValue.internal?.retrySetupOnBoot ? Infinity : 1,
-            // 250ms initial backoff
-            startingDelay: 250,
+            numOfAttempts: setupAttempts,
+            // 1s initial backoff
+            startingDelay: 1000,
             // 5m max backoff
             maxDelay: 60000 * 5,
             timeMultiple: 2,
@@ -566,7 +568,7 @@ export class FleetPlugin
             jitter: 'full',
             retry: (error: any, attemptCount: number) => {
               const summary = `Fleet setup attempt ${attemptCount} failed, will retry after backoff`;
-              logger.debug(summary, { error: { message: error } });
+              logger.warn(summary, { error: { message: error } });
 
               this.fleetStatus$.next({
                 level: ServiceStatusLevels.available,
@@ -586,7 +588,9 @@ export class FleetPlugin
           summary: 'Fleet is available',
         });
       } catch (error) {
-        logger.warn('Fleet setup failed', { error: { message: error } });
+        logger.warn(`Fleet setup failed after ${setupAttempts} attempts`, {
+          error: { message: error },
+        });
 
         this.fleetStatus$.next({
           // As long as Fleet has a dependency on EPR, we can't reliably set Kibana status to `unavailable` here.