Do not overwrite client span if incoming request has "uber-trace-id" header

Author: rafis

instrument-starters/opentracing-spring-cloud-gateway-starter/src/main/java/io/opentracing/contrib/spring/cloud/gateway/TraceRequestHttpHeadersFilter.java

@@ -66,7 +66,9 @@ public HttpHeaders filter(HttpHeaders input, ServerWebExchange exchange) {
     } catch (Exception ignore) {
       log.error("TraceRequestHttpHeadersFilter error", ignore);
     }
-    headersWithInput.addAll(input);
+    for (Map.Entry<String, List<String>> entry : input.entrySet()) {
+      headersWithInput.addIfAbsent(entry.getKey(), entry.getValue().get(0));
+    }
     addHeadersWithInput(builder, headersWithInput);
     return headersWithInput;
   }