[DRAFT] NOT MERGING POC: Send JVM runtime metrics via OTLP using OTel-native naming

dd-java-agent/agent-bootstrap/src/main/java/datadog/trace/bootstrap/Agent.java

@@ -26,6 +26,7 @@
 import datadog.instrument.utils.ClassLoaderValue;
 import datadog.metrics.api.statsd.StatsDClientManager;
 import datadog.trace.api.Config;
+import datadog.trace.api.InstrumenterConfig;
 import datadog.trace.api.Platform;
 import datadog.trace.api.WithGlobalTracer;
 import datadog.trace.api.appsec.AppSecEventTracker;
@@ -846,6 +847,17 @@ private static synchronized void installDatadogTracer(
       initTelemetry.onFatalError(ex);
     }
 
+    // OTLP runtime metrics start unconditionally when both DD_METRICS_OTEL_ENABLED
+    // and DD_RUNTIME_METRICS_ENABLED are set, regardless of whether the application
+    // imports the OTel API. The OTLP exporter (OtlpMetricsService) was just started
+    // by CoreTracer above; this registers the JVM metric callbacks with
+    // OtelMeterProvider so the periodic export has data to collect. Done here
+    // (not in startJmx, which is delayed 15s) so callbacks are registered before
+    // the exporter's first scheduled flush — otherwise short-lived tests miss data.
+    if (Config.get().isRuntimeMetricsEnabled() && InstrumenterConfig.get().isMetricsOtelEnabled()) {
+      startOtlpRuntimeMetrics();
+    }
+
     StaticEventLogger.end("GlobalTracer");
   }
 
@@ -989,6 +1001,27 @@ private static synchronized void initializeJmxSystemAccessProvider(
     }
   }
 
+  /**
+   * Registers OTLP runtime metric callbacks (JVM heap, CPU, threads, classes, etc.) with the
+   * agent's OtelMeterProvider. The periodic OTLP exporter started by CoreTracer then collects and
+   * exports them — this is the same pattern Node and .NET use to start their runtime metrics
+   * unconditionally during tracer init, independent of any app-side OTel API usage.
+   */
+  private static synchronized void startOtlpRuntimeMetrics() {
+    final ClassLoader contextLoader = Thread.currentThread().getContextClassLoader();
+    try {
+      Thread.currentThread().setContextClassLoader(AGENT_CLASSLOADER);
+      final Class<?> jvmOtlpClass =
+          AGENT_CLASSLOADER.loadClass("datadog.opentelemetry.shim.metrics.JvmOtlpRuntimeMetrics");
+      final Method startMethod = jvmOtlpClass.getMethod("start");
+      startMethod.invoke(null);
+    } catch (final Throwable ex) {
+      log.error("Throwable thrown while starting OTLP runtime metrics", ex);
+    } finally {
+      safelySetContextClassLoader(contextLoader);
+    }
+  }
+
   private static synchronized void startJmxFetch() {
     final ClassLoader contextLoader = Thread.currentThread().getContextClassLoader();
     try {

dd-java-agent/agent-bootstrap/src/main/resources/META-INF/native-image/com.datadoghq/dd-java-agent/reflect-config.json

@@ -218,5 +218,11 @@
     "methods": [
       {"name": "<init>", "parameterTypes": []}
     ]
+  },
+  {
+    "name": "datadog.trace.bootstrap.otel.shim.metrics.JvmOtlpRuntimeMetrics",
+    "methods": [
+      {"name": "start", "parameterTypes": []}
+    ]
   }
 ]