Restore customization of HTTP trace request headers

This commit restore the use of the `postProcessRequestHeaders` method
when overridden in a custom HttpExchangeTracer implementation.

Closes gh-13924

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/trace/http/HttpExchangeTracer.java

@@ -127,8 +127,11 @@ public class HttpExchangeTracer {
 
 		@Override
 		public Map<String, List<String>> getHeaders() {
-			return getHeadersIfIncluded(Include.REQUEST_HEADERS,
-					this.delegate::getHeaders, this::includedHeader);
+			Map<String, List<String>> headers = getHeadersIfIncluded(
+					Include.REQUEST_HEADERS, this.delegate::getHeaders,
+					this::includedHeader);
+			postProcessRequestHeaders(headers);
+			return headers;
 		}
 
 		private boolean includedHeader(String name) {

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/trace/http/HttpExchangeTracerTests.java

@@ -29,6 +29,8 @@ import org.junit.Test;
 
 import org.springframework.boot.actuate.trace.http.HttpTrace.Request;
 import org.springframework.http.HttpHeaders;
+import org.springframework.util.LinkedMultiValueMap;
+import org.springframework.util.MultiValueMap;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.mockito.BDDMockito.given;
@@ -89,6 +91,19 @@ public class HttpExchangeTracerTests {
 		assertThat(request.getHeaders()).containsOnlyKeys(HttpHeaders.ACCEPT);
 	}
 
+	@Test
+	public void requestHeadersCanBeCustomized() {
+		MultiValueMap<String, String> headers = new LinkedMultiValueMap<>();
+		headers.add("to-remove", "test");
+		headers.add("test", "value");
+		HttpTrace trace = new RequestHeadersFilterHttpExchangeTracer()
+				.receivedRequest(createRequest(headers));
+		Request request = trace.getRequest();
+		assertThat(request.getHeaders()).containsOnlyKeys("test", "to-add");
+		assertThat(request.getHeaders().get("test")).containsExactly("value");
+		assertThat(request.getHeaders().get("to-add")).containsExactly("42");
+	}
+
 	@Test
 	public void authorizationHeaderIsNotIncludedByDefault() {
 		HttpTrace trace = new HttpExchangeTracer(EnumSet.of(Include.REQUEST_HEADERS))
@@ -332,4 +347,19 @@ public class HttpExchangeTracerTests {
 		return output.toString();
 	}
 
+	private static class RequestHeadersFilterHttpExchangeTracer
+			extends HttpExchangeTracer {
+
+		RequestHeadersFilterHttpExchangeTracer() {
+			super(EnumSet.of(Include.REQUEST_HEADERS));
+		}
+
+		@Override
+		protected void postProcessRequestHeaders(Map<String, List<String>> headers) {
+			headers.remove("to-remove");
+			headers.putIfAbsent("to-add", Collections.singletonList("42"));
+		}
+
+	}
+
 }

spring-boot-project/spring-boot-docs/src/main/asciidoc/production-ready-features.adoc

@@ -1915,7 +1915,8 @@ endpoint and obtain basic information about the last 100 request-response exchan
 [[production-ready-http-tracing-custom]]
 === Custom HTTP tracing
 To customize the items that are included in each trace, use the
-`management.trace.http.include` configuration property.
+`management.trace.http.include` configuration property. For advanced customization,
+consider registering your own `HttpExchangeTracer` implementation.
 
 By default, an `InMemoryHttpTraceRepository` that stores traces for the last 100
 request-response exchanges is used. If you need to expand the capacity, you can define