[orchagent] Do not pollute logs when unknown field is found

This will prevent future unittest failing since there is syslog rate
limitting in place, causing message drop when large amount of messages
is arriving

orchagent/port/porthlpr.cpp

@@ -6,6 +6,7 @@
 #include <unordered_map>
 #include <string>
 #include <exception>
+#include <set>
 
 #include <boost/algorithm/string.hpp>
 
@@ -777,6 +778,8 @@ bool PortHelper::parsePortConfig(PortConfig &port) const
 {
     SWSS_LOG_ENTER();
 
+    std::set<std::string> unknown;
+
     for (const auto &cit : port.fieldValueMap)
     {
         const auto &field = cit.first;
@@ -1029,6 +1032,11 @@ bool PortHelper::parsePortConfig(PortConfig &port) const
         }
         else
         {
+            if (unknown.find(field) != unknown.end())
+                continue; // do not pollute logs with the same unknown field
+
+            unknown.insert(field);
+
             SWSS_LOG_WARN("Unknown field(%s): skipping ...", field.c_str());
         }
     }