implemented 09-system-metrics.md: added system metrics: introduced system monitoring with a metrics collector, REST API endpoint, Thymeleaf UI rendering, and lifecycle management

src/main/kotlin/de/hoennig/gittally/server/ServerMetricsLifecycle.kt

@@ -0,0 +1,28 @@
+package de.hoennig.gittally.server
+
+import de.hoennig.gittally.metrics.SystemMetricsCollector
+import jakarta.annotation.PreDestroy
+import org.springframework.boot.context.event.ApplicationReadyEvent
+import org.springframework.context.annotation.Profile
+import org.springframework.context.event.EventListener
+import org.springframework.stereotype.Component
+
+/**
+ * Starts the system-metrics sampling loop once the server context is ready and
+ * stops it on shutdown. Only in the `server` profile, like [ServerWatcherLifecycle].
+ */
+@Component
+@Profile("server")
+class ServerMetricsLifecycle(
+    private val collector: SystemMetricsCollector,
+) {
+    @EventListener(ApplicationReadyEvent::class)
+    fun onApplicationReady() {
+        collector.start()
+    }
+
+    @PreDestroy
+    fun onShutdown() {
+        collector.stop()
+    }
+}