fix: px protocol decode - do not treat missing response field as error (

#3055)

* Fix missing response field of PeerExchange RPC treated as error.
* Fix PX metrics from gauge to counter for better dashboard stats

waku/waku_peer_exchange/protocol.nim

@@ -20,9 +20,10 @@ declarePublicGauge waku_px_peers_received_total,
   "number of ENRs received via peer exchange"
 declarePublicGauge waku_px_peers_received_unknown,
   "number of previously unknown ENRs received via peer exchange"
-declarePublicGauge waku_px_peers_sent, "number of ENRs sent to peer exchange requesters"
+declarePublicCounter waku_px_peers_sent,
+  "number of ENRs sent to peer exchange requesters"
 declarePublicGauge waku_px_peers_cached, "number of peer exchange peer ENRs cached"
-declarePublicGauge waku_px_errors, "number of peer exchange errors", ["type"]
+declarePublicCounter waku_px_errors, "number of peer exchange errors", ["type"]
 
 logScope:
   topics = "waku peer_exchange"

waku/waku_peer_exchange/rpc_codec.nim

@@ -103,8 +103,9 @@ proc decode*(T: type PeerExchangeRpc, buffer: seq[byte]): ProtobufResult[T] =
 
   var responseBuffer: seq[byte]
   if not ?pb.getField(2, responseBuffer):
-    return err(ProtobufError.missingRequiredField("response"))
-
-  rpc.response = ?PeerExchangeResponse.decode(responseBuffer)
+    rpc.response =
+      PeerExchangeResponse(status_code: PeerExchangeResponseStatusCode.UNKNOWN)
+  else:
+    rpc.response = ?PeerExchangeResponse.decode(responseBuffer)
 
   ok(rpc)