Modernize services to use Symfony autowiring, autoconfiguration, and PHP 8 features

Problem/Motivation

Modernize the prometheus_exporter module to use Symfony container autowiring and autoconfiguration, reducing boilerplate in service definitions and improving IDE support.

Steps to reproduce

Proposed resolution

• Add _defaults with autowire: true and autoconfigure: true to services.yml files
• Remove explicit arguments from service definitions (autowiring handles injection)
• Remove event_subscriber tags (autoconfigure adds them automatically)
• Add class-based service aliases (e.g., Drupal\prometheus_exporter\MetricsCollectorManager: '@prometheus_exporter.metrics_collector_manager')
• Replace string service IDs with ::class constants in $container->get() calls
• Refactor classes to use constructor property promotion, typed properties, and return types on create() methods
• Use EntityTypeManagerInterface instead of storing entity storage directly

Remaining tasks

User interface changes

API changes

Data model changes