Prevent NPE when using otel metrics bridge asynchronously to agent start

CHANGELOG.asciidoc

@@ -31,6 +31,10 @@ Use subheadings with the "=====" level for adding notes for unreleased changes:
 
 === Unreleased
 
+[float]
+===== Bug fixes
+* Prevent NPE in OpenTelemetry metrics bridge in case of asynchronous agent start - {pull}3880[#3880]
+
 [[release-notes-1.x]]
 === Java Agent version 1.x
 

apm-agent-plugins/apm-opentelemetry/apm-opentelemetry-embedded-metrics-sdk/src/main/java/co/elastic/apm/agent/embeddedotel/EmbeddedSdkManager.java

@@ -18,11 +18,12 @@
  */
 package co.elastic.apm.agent.embeddedotel;
 
-import co.elastic.apm.agent.tracer.AbstractLifecycleListener;
 import co.elastic.apm.agent.embeddedotel.proxy.ProxyMeterProvider;
 import co.elastic.apm.agent.sdk.logging.Logger;
 import co.elastic.apm.agent.sdk.logging.LoggerFactory;
+import co.elastic.apm.agent.tracer.AbstractLifecycleListener;
 import co.elastic.apm.agent.tracer.Tracer;
+import io.opentelemetry.api.metrics.MeterProvider;
 import io.opentelemetry.sdk.metrics.SdkMeterProvider;
 import io.opentelemetry.sdk.metrics.SdkMeterProviderBuilder;
 
@@ -65,6 +66,10 @@ public ProxyMeterProvider getMeterProvider() {
         if (sdkInstance == null) {
             startSdk();
         }
+        if (sdkInstance == null) {
+            logger.warn("Returning NoOp-MeterProvider because OpenTelemetry metrics SDK could not be initialized!");
+            return new ProxyMeterProvider(MeterProvider.noop());
+        }
         return new ProxyMeterProvider(sdkInstance);
     }
 
@@ -78,7 +83,11 @@ synchronized void reset() {
     }
 
     private synchronized void startSdk() {
-        if (isShutdown || sdkInstance != null || tracer == null) {
+        if (isShutdown || sdkInstance != null) {
+            return;
+        }
+        if (tracer == null) {
+            logger.warn("Cannot initialize OpenTelemetry metrics SDK because tracer has not started yet");
             return;
         }
         logger.debug("Starting embedded OpenTelemetry metrics SDK");

apm-agent-plugins/apm-opentelemetry/apm-opentelemetry-embedded-metrics-sdk/src/test/java/co/elastic/apm/agent/embeddedotel/EmbeddedSdkManagerTest.java

@@ -0,0 +1,22 @@
+package co.elastic.apm.agent.embeddedotel;
+
+import co.elastic.apm.agent.embeddedotel.proxy.ProxyMeter;
+import co.elastic.apm.agent.tracer.Tracer;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class EmbeddedSdkManagerTest {
+
+    /**
+     * The instrumentation of the agent is performed before {@link EmbeddedSdkManager#init(Tracer)} is invoked.
+     * This means if the agent is started asynchronously, it can happen that {@link EmbeddedSdkManager#getMeterProvider()}
+     * is invoked before the tracer has been provided.
+     * This test verifies that in that case no exception occurs and a noop-meter implementation is used.
+     */
+    @Test
+    public void ensureNoExceptionOnMissingTracer() throws Exception {
+        ProxyMeter meter = new EmbeddedSdkManager().getMeterProvider().get("foobar");
+        assertThat(meter.getDelegate()).isInstanceOf(Class.forName("io.opentelemetry.api.metrics.DefaultMeter"));
+    }
+}