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

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

@@ -86,7 +86,7 @@ private void addHeadersWithInput(ServerHttpRequest.Builder builder,
         .entrySet()) {
       String key = entry.getKey();
       List<String> value = entry.getValue();
-      headersWithInput.put(key, value);
+      headersWithInput.putIfAbsent(key, value);
     }
   }