fix(openmetrics): Multiple exporters per app

[SystemKeeper]

When apps specify multiple exporters, the ExportManager crashes:

Nachricht: Illegal offset type in isset or empty
Datei: /var/www/html/lib/private/OpenMetrics/ExporterManager.php
Zeile: 67

#0 /var/www/html/core/Controller/OpenMetricsController.php(80): OC\OpenMetrics\ExporterManager->export()
#1 /var/www/html/lib/public/AppFramework/Http/StreamTraversableResponse.php(46): OC\Core\Controller\OpenMetricsController->generate()
#2 /var/www/html/lib/private/AppFramework/App.php(210): OCP\AppFramework\Http\StreamTraversableResponse->callback(Object(OC\AppFramework\Http\Output))
#3 /var/www/html/lib/private/Route/Router.php(321): OC\AppFramework\App::main('OC\\Core\\Control...', 'export', Object(OC\AppFramework\DependencyInjection\DIContainer), Array)
#4 /var/www/html/lib/base.php(1125): OC\Route\Router->match('/metrics')
#5 /var/www/html/index.php(25): OC::handleRequest()
#6 {main}

Not really sure how to easily add a test for this.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

[SystemKeeper]

/backport to stable33