Added log for correlation id

src/main/java/com/example/http/reactive/handler/LogWebFilter.java

+package com.example.http.reactive.handler;
+
+import java.util.List;
+import lombok.extern.slf4j.Slf4j;
+import org.springframework.stereotype.Component;
+import org.springframework.web.server.ServerWebExchange;
+import org.springframework.web.server.WebFilter;
+import org.springframework.web.server.WebFilterChain;
+import reactor.core.publisher.Mono;
+
+@Slf4j
+@Component
+public class LogWebFilter implements WebFilter {
+
+  private static String correlationIdHeader = "X-Correlation-Id";
+
+  @Override
+  public Mono<Void> filter(ServerWebExchange exchange, WebFilterChain chain) {
+    if (exchange.getRequest().getHeaders().containsKey(correlationIdHeader)) {
+      List<String> correlationId = exchange.getRequest().getHeaders().get(correlationIdHeader);
+      log.info("External Correlation Id:[{}]", correlationId);
+      exchange.getResponse().getHeaders().addAll(correlationIdHeader, correlationId);
+    }
+    return chain.filter(exchange);
+  }
+}