fix: inject Dev UI tracer into core engine for embedded telemetry

Author: hemasekhar-p

core/src/main/java/com/google/adk/telemetry/Tracing.java

@@ -133,14 +133,20 @@ public class Tracing {
       AttributeKey.stringKey("gcp.vertex.agent.data");
 
   @SuppressWarnings("NonFinalStaticField")
-  private static Tracer tracer = GlobalOpenTelemetry.getTracer("gcp.vertex.agent");
+  private static volatile Tracer tracer = GlobalOpenTelemetry.getTracer("gcp.vertex.agent");
 
   private static final boolean CAPTURE_MESSAGE_CONTENT_IN_SPANS =
       Boolean.parseBoolean(
           System.getenv().getOrDefault("ADK_CAPTURE_MESSAGE_CONTENT_IN_SPANS", "true"));
 
   private Tracing() {}
 
+  public static void setTracer(Tracer customTracer) {
+    if (customTracer != null) {
+      tracer = customTracer;
+    }
+  }
+
   private static void setInvocationAttributes(
       Span span, InvocationContext invocationContext, String eventId) {
     span.setAttribute(ADK_INVOCATION_ID, invocationContext.invocationId());

dev/src/main/java/com/google/adk/web/config/OpenTelemetryConfig.java

@@ -16,6 +16,7 @@
 
 package com.google.adk.web.config;
 
+import com.google.adk.telemetry.Tracing;
 import com.google.adk.web.service.ApiServerSpanExporter;
 import io.opentelemetry.api.OpenTelemetry;
 import io.opentelemetry.api.common.AttributeKey;
@@ -64,13 +65,17 @@ public OpenTelemetry openTelemetrySdk(SdkTracerProvider sdkTracerProvider) {
       // If we get here, it's already set, so just return a new instance without global
       // registration
       otelLog.debug("OpenTelemetry already registered globally, creating non-global instance.");
+      OpenTelemetrySdk sdk =
+          OpenTelemetrySdk.builder().setTracerProvider(sdkTracerProvider).build();
+      Tracing.setTracer(sdk.getTracer("gcp.vertex.agent"));
       return OpenTelemetrySdk.builder().setTracerProvider(sdkTracerProvider).build();
     } catch (IllegalStateException e) {
       // GlobalOpenTelemetry hasn't been set yet, safe to register globally
       otelLog.debug("Registering OpenTelemetry globally.");
       OpenTelemetrySdk otelSdk =
           OpenTelemetrySdk.builder().setTracerProvider(sdkTracerProvider).buildAndRegisterGlobal();
       Runtime.getRuntime().addShutdownHook(new Thread(otelSdk::close));
+      Tracing.setTracer(otelSdk.getTracer("gcp.vertex.agent"));
       return otelSdk;
     }
   }