Ensures Zipkin handlers order last (after redacters etc) (#1648)

This is more prevention of a problem, then seeing one in practice.
Technically, a TracingCustomizer can order things backwards, and it hit
me in a test env (not sleuth).

spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/autoconfig/TraceAutoConfiguration.java

@@ -16,7 +16,9 @@
 
 package org.springframework.cloud.sleuth.autoconfig;
 
+import java.util.ArrayList;
 import java.util.Collections;
+import java.util.Comparator;
 import java.util.List;
 
 import brave.CurrentSpanCustomizer;
@@ -37,6 +39,7 @@ import zipkin2.Span;
 import zipkin2.reporter.InMemoryReporterMetrics;
 import zipkin2.reporter.Reporter;
 import zipkin2.reporter.ReporterMetrics;
+import zipkin2.reporter.brave.ZipkinSpanHandler;
 import zipkin2.reporter.metrics.micrometer.MicrometerReporterMetrics;
 
 import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
@@ -85,6 +88,22 @@ public class TraceAutoConfiguration {
 	 */
 	public static final String DEFAULT_SERVICE_NAME = "default";
 
+	/**
+	 * Sort Zipkin Handlers last, so that redactions etc happen prior.
+	 */
+	static final Comparator<SpanHandler> SPAN_HANDLER_COMPARATOR = (o1, o2) -> {
+		if (o1 instanceof ZipkinSpanHandler) {
+			if (o2 instanceof ZipkinSpanHandler) {
+				return 0;
+			}
+			return 1;
+		}
+		else if (o2 instanceof ZipkinSpanHandler) {
+			return -1;
+		}
+		return 0;
+	};
+
 	@Bean
 	@ConditionalOnMissingBean
 	// NOTE: stable bean name as might be used outside sleuth
@@ -117,9 +136,21 @@ public class TraceAutoConfiguration {
 				customizer.customize(builder);
 			}
 		}
+
+		reorderZipkinHandlersLast(builder);
 		return builder.build();
 	}
 
+	private void reorderZipkinHandlersLast(Tracing.Builder builder) {
+		List<SpanHandler> configuredSpanHandlers = new ArrayList<>(
+				builder.spanHandlers());
+		configuredSpanHandlers.sort(SPAN_HANDLER_COMPARATOR);
+		builder.clearSpanHandlers();
+		for (SpanHandler spanHandler : configuredSpanHandlers) {
+			builder.addSpanHandler(spanHandler);
+		}
+	}
+
 	@Bean(name = TRACER_BEAN_NAME)
 	@ConditionalOnMissingBean
 	Tracer tracer(Tracing tracing) {

spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/autoconfig/TraceAutoConfigurationTests.java

@@ -16,6 +16,7 @@
 
 package org.springframework.cloud.sleuth.autoconfig;
 
+import java.util.ArrayList;
 import java.util.List;
 
 import brave.Tracing;
@@ -23,6 +24,7 @@ import brave.baggage.BaggageField;
 import brave.baggage.BaggagePropagation;
 import brave.baggage.BaggagePropagationConfig.SingleBaggageField;
 import brave.baggage.BaggagePropagationCustomizer;
+import brave.handler.SpanHandler;
 import brave.propagation.B3Propagation;
 import brave.propagation.B3SinglePropagation;
 import brave.propagation.ExtraFieldPropagation;
@@ -38,6 +40,7 @@ import org.junit.Test;
 import zipkin2.reporter.InMemoryReporterMetrics;
 import zipkin2.reporter.Reporter;
 import zipkin2.reporter.ReporterMetrics;
+import zipkin2.reporter.brave.ZipkinSpanHandler;
 import zipkin2.reporter.metrics.micrometer.MicrometerReporterMetrics;
 
 import org.springframework.beans.factory.annotation.Autowired;
@@ -47,11 +50,33 @@ import org.springframework.boot.test.context.runner.ApplicationContextRunner;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.mock;
+import static org.springframework.cloud.sleuth.autoconfig.TraceAutoConfiguration.SPAN_HANDLER_COMPARATOR;
+
 public class TraceAutoConfigurationTests {
 
 	private final ApplicationContextRunner contextRunner = new ApplicationContextRunner()
 			.withConfiguration(AutoConfigurations.of(TraceAutoConfiguration.class));
 
+	@Test
+	public void span_handler_comparator() {
+		SpanHandler handler1 = mock(SpanHandler.class);
+		SpanHandler handler2 = mock(SpanHandler.class);
+		ZipkinSpanHandler zipkin1 = mock(ZipkinSpanHandler.class);
+		ZipkinSpanHandler zipkin2 = mock(ZipkinSpanHandler.class);
+
+		ArrayList<SpanHandler> spanHandlers = new ArrayList<>();
+		spanHandlers.add(handler1);
+		spanHandlers.add(zipkin1);
+		spanHandlers.add(handler2);
+		spanHandlers.add(zipkin2);
+
+		spanHandlers.sort(SPAN_HANDLER_COMPARATOR);
+
+		assertThat(spanHandlers).containsExactly(handler1, handler2, zipkin1, zipkin2);
+	}
+
 	@Test
 	public void should_apply_micrometer_reporter_metrics_when_meter_registry_bean_present() {
 		this.contextRunner.withUserConfiguration(WithMeterRegistry.class)