Don't back off on user-supplied SpanHandlers

Instead the auto-configuration now backs off only on ZipkinSpanHandler.

Closes gh-31273

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/tracing/zipkin/ZipkinConfigurations.java

@@ -16,7 +16,6 @@
 
 package org.springframework.boot.actuate.autoconfigure.tracing.zipkin;
 
-import brave.handler.SpanHandler;
 import io.opentelemetry.exporter.zipkin.ZipkinSpanExporter;
 import zipkin2.Span;
 import zipkin2.codec.BytesEncoder;
@@ -87,8 +86,8 @@ class ZipkinConfigurations {
 		@Bean
 		@ConditionalOnMissingBean
 		@ConditionalOnBean(Reporter.class)
-		SpanHandler zipkinSpanHandler(Reporter<Span> spanReporter) {
-			return ZipkinSpanHandler.newBuilder(spanReporter).build();
+		ZipkinSpanHandler zipkinSpanHandler(Reporter<Span> spanReporter) {
+			return (ZipkinSpanHandler) ZipkinSpanHandler.newBuilder(spanReporter).build();
 		}
 
 	}

spring-boot-project/spring-boot-actuator-autoconfigure/src/test/java/org/springframework/boot/actuate/autoconfigure/tracing/zipkin/ZipkinConfigurationsBraveConfigurationTests.java

@@ -21,6 +21,7 @@ import org.junit.jupiter.api.Test;
 import org.mockito.Mockito;
 import zipkin2.Span;
 import zipkin2.reporter.Reporter;
+import zipkin2.reporter.brave.ZipkinSpanHandler;
 
 import org.springframework.boot.actuate.autoconfigure.tracing.zipkin.ZipkinConfigurations.BraveConfiguration;
 import org.springframework.boot.autoconfigure.AutoConfigurations;
@@ -44,28 +45,38 @@ class ZipkinConfigurationsBraveConfigurationTests {
 	@Test
 	void shouldSupplyBeans() {
 		this.contextRunner.withUserConfiguration(ReporterConfiguration.class)
-				.run((context) -> assertThat(context).hasSingleBean(SpanHandler.class));
+				.run((context) -> assertThat(context).hasSingleBean(ZipkinSpanHandler.class));
 	}
 
 	@Test
 	void shouldNotSupplySpanHandlerIfReporterIsMissing() {
-		this.contextRunner.run((context) -> assertThat(context).doesNotHaveBean(SpanHandler.class));
+		this.contextRunner.run((context) -> assertThat(context).doesNotHaveBean(ZipkinSpanHandler.class));
 	}
 
 	@Test
 	void shouldNotSupplyIfZipkinReporterBraveIsNotOnClasspath() {
 		this.contextRunner.withClassLoader(new FilteredClassLoader("zipkin2.reporter.brave"))
 				.withUserConfiguration(ReporterConfiguration.class)
-				.run((context) -> assertThat(context).doesNotHaveBean(SpanHandler.class));
+				.run((context) -> assertThat(context).doesNotHaveBean(ZipkinSpanHandler.class));
 
 	}
 
 	@Test
 	void shouldBackOffOnCustomBeans() {
-		this.contextRunner.withUserConfiguration(CustomConfiguration.class).run((context) -> {
-			assertThat(context).hasBean("customSpanHandler");
-			assertThat(context).hasSingleBean(SpanHandler.class);
-		});
+		this.contextRunner.withUserConfiguration(ReporterConfiguration.class, CustomConfiguration.class)
+				.run((context) -> {
+					assertThat(context).hasBean("customZipkinSpanHandler");
+					assertThat(context).hasSingleBean(ZipkinSpanHandler.class);
+				});
+	}
+
+	@Test
+	void shouldSupplyZipkinSpanHandlerWithCustomSpanHandler() {
+		this.contextRunner.withUserConfiguration(ReporterConfiguration.class, CustomSpanHandlerConfiguration.class)
+				.run((context) -> {
+					assertThat(context).hasBean("customSpanHandler");
+					assertThat(context).hasSingleBean(ZipkinSpanHandler.class);
+				});
 	}
 
 	@Configuration(proxyBeanMethods = false)
@@ -82,6 +93,17 @@ class ZipkinConfigurationsBraveConfigurationTests {
 	@Configuration(proxyBeanMethods = false)
 	private static class CustomConfiguration {
 
+		@Bean
+		@SuppressWarnings("unchecked")
+		ZipkinSpanHandler customZipkinSpanHandler() {
+			return (ZipkinSpanHandler) ZipkinSpanHandler.create(Mockito.mock(Reporter.class));
+		}
+
+	}
+
+	@Configuration(proxyBeanMethods = false)
+	private static class CustomSpanHandlerConfiguration {
+
 		@Bean
 		SpanHandler customSpanHandler() {
 			return Mockito.mock(SpanHandler.class);