multi: set the InboundFee on ChannelEdgePolicy

In this commit, we make sure to set the new field wherever appropriate.
This will be any place where the ChannelEdgePolicy is constructed other
than its disk deserialisation.

discovery/gossiper.go

@@ -3260,6 +3260,26 @@ func (d *AuthenticatedGossiper) handleChanUpdate(ctx context.Context,
 		ExtraOpaqueData:           upd.ExtraOpaqueData,
 	}
 
+	// Extract the inbound fee from the ExtraOpaqueData, if present.
+	//
+	// TODO(elle): this can be removed once we define the optional TLV
+	// field on the lnwire.ChannelUpdate itself.
+	var inboundFee lnwire.Fee
+	typeMap, err := upd.ExtraOpaqueData.ExtractRecords(&inboundFee)
+	if err != nil {
+		rErr := fmt.Errorf("%w: %w", graphdb.ErrParsingExtraTLVBytes,
+			err)
+
+		log.Error(rErr)
+		nMsg.err <- rErr
+		return nil, false
+	}
+
+	val, ok := typeMap[lnwire.FeeRecordType]
+	if ok && val == nil {
+		update.InboundFee = fn.Some(inboundFee)
+	}
+
 	if err := d.cfg.Graph.UpdateEdge(update, ops...); err != nil {
 		if graph.IsError(
 			err, graph.ErrOutdated,