Refactor docs; Add 'ipng_source_tag', add udp listener for nginx-ipng-stats plugin

proto/logtail.proto

@@ -38,6 +38,7 @@ message Filter {
   TorFilter       tor                = 8;  // restrict to TOR / non-TOR clients
   optional int32  asn_number         = 9;  // filter by client ASN
   StatusOp        asn_op             = 10; // operator for asn_number; ignored when unset
+  optional string ipng_source_tag    = 13; // filter by nginx source tag
 }
 
 enum GroupBy {
@@ -46,6 +47,7 @@ enum GroupBy {
   REQUEST_URI  = 2;
   HTTP_RESPONSE = 3;
   ASN_NUMBER   = 4;
+  SOURCE_TAG   = 5;
 }
 
 enum Window {