Fix #261:Add support for traceparent headers in DefaultRestClient (#262)

* Fix #261:Add support for traceparent headers in DefaultRestClient

rest-client-base/pom.xml

@@ -61,6 +61,12 @@
             <artifactId>netty-http-authenticator</artifactId>
             <version>1.5</version>
         </dependency>
+
+        <dependency>
+            <groupId>io.opentelemetry</groupId>
+            <artifactId>opentelemetry-api</artifactId>
+        </dependency>
+
     </dependencies>
 
     <build>

rest-client-base/src/main/java/com/wultra/core/rest/client/base/DefaultRestClient.java

@@ -212,6 +212,7 @@ private void initializeWebClient() throws RestClientException {
                         });
             });
         }
+        builder.filter(TraceparentFilterFunction.handleTraceparentContext());
 
         final ReactorClientHttpConnector connector = new ReactorClientHttpConnector(httpClient);
         webClient = builder.baseUrl(config.getBaseUrl()).clientConnector(connector).build();

rest-client-base/src/main/java/com/wultra/core/rest/client/base/TraceparentFilterFunction.java

@@ -0,0 +1,86 @@
+/*
+ * Copyright 2024 Wultra s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.wultra.core.rest.client.base;
+
+import io.opentelemetry.api.trace.Span;
+import io.opentelemetry.api.trace.SpanContext;
+import io.opentelemetry.api.trace.TraceFlags;
+import org.springframework.web.reactive.function.client.ClientRequest;
+import org.springframework.web.reactive.function.client.ClientResponse;
+import org.springframework.web.reactive.function.client.ExchangeFilterFunction;
+import org.springframework.web.reactive.function.client.ExchangeFunction;
+import reactor.core.publisher.Mono;
+
+/**
+ * An ExchangeFilterFunction for adding traceparent header to WebClient requests.
+ * It also propagates trace ID through Reactor context and MDC for logging purposes.
+ *
+ * @author Jan Dusil, [email protected]
+ */
+public class TraceparentFilterFunction implements ExchangeFilterFunction {
+
+    private static final String TRACEPARENT_HEADER_KEY = "traceparent";
+
+    /**
+     * Applies the filter to the given ClientRequest and ExchangeFunction.
+     *
+     * @param request The client request.
+     * @param next    The next exchange function in the chain.
+     * @return a Mono<ClientResponse> after applying the filter.
+     */
+    @Override
+    public Mono<ClientResponse> filter(final ClientRequest request, final ExchangeFunction next) {
+        return next.exchange(addTraceparentHeader(request));
+    }
+
+    /**
+     * Adds a traceparent header to the ClientRequest if a current span context is available.
+     *
+     * @param request The client request.
+     * @return a modified ClientRequest with the traceparent header added.
+     */
+    private ClientRequest addTraceparentHeader(final ClientRequest request) {
+        final Span currentSpan = Span.current();
+        if (currentSpan != null) {
+            final SpanContext spanContext = currentSpan.getSpanContext();
+            if (spanContext != null) {
+                final String traceId = spanContext.getTraceId();
+                /* The parentId of the next server Span will be the current spanId */
+                final String parentId = spanContext.getSpanId();
+                final TraceFlags traceFlags = spanContext.getTraceFlags();
+                if (traceId != null && parentId != null && traceFlags != null) {
+                    final String headerValue = String.format("00-%s-%s-%s",
+                            traceId,
+                            parentId,
+                            traceFlags.asHex());
+                    return ClientRequest.from(request)
+                            .headers(headers -> headers.set(TRACEPARENT_HEADER_KEY, headerValue))
+                            .build();
+                }
+            }
+        }
+        return request;
+    }
+
+    /**
+     * Factory method to create an instance of TraceparentFilterFunction.
+     *
+     * @return a new instance of TraceparentFilterFunction.
+     */
+    public static ExchangeFilterFunction handleTraceparentContext() {
+        return new TraceparentFilterFunction();
+    }
+}