routing+channeldb: send payment metadata from invoice

lnrpc/lightning.proto

@@ -2792,6 +2792,9 @@ message Hop {
     to drop off at each hop within the onion.
     */
     map<uint64, bytes> custom_records = 11;
+
+    // The payment metadata to send along with the payment to the payee.
+    bytes metadata = 13;
 }
 
 message MPPRecord {

lnrpc/lightning.swagger.json

@@ -4616,6 +4616,11 @@
             "format": "byte"
           },
           "description": "An optional set of key-value TLV records. This is useful within the context\nof the SendToRoute call as it allows callers to specify arbitrary K-V pairs\nto drop off at each hop within the onion."
+        },
+        "metadata": {
+          "type": "string",
+          "format": "byte",
+          "description": "The payment metadata to send along with the payment to the payee."
         }
       }
     },

lnrpc/routerrpc/router.swagger.json

@@ -819,6 +819,11 @@
             "format": "byte"
           },
           "description": "An optional set of key-value TLV records. This is useful within the context\nof the SendToRoute call as it allows callers to specify arbitrary K-V pairs\nto drop off at each hop within the onion."
+        },
+        "metadata": {
+          "type": "string",
+          "format": "byte",
+          "description": "The payment metadata to send along with the payment to the payee."
         }
       }
     },

lnrpc/routerrpc/router_backend.go

@@ -444,6 +444,7 @@ func (r *RouterBackend) MarshallRoute(route *route.Route) (*lnrpc.Route, error)
 			CustomRecords: hop.CustomRecords,
 			TlvPayload:    !hop.LegacyPayload,
 			MppRecord:     mpp,
+			Metadata:      hop.Metadata,
 		}
 		incomingAmt = hop.AmtToForward
 	}
@@ -766,6 +767,7 @@ func (r *RouterBackend) extractIntentFromSendRequest(
 		payIntent.DestFeatures = payReq.Features
 		payIntent.PaymentAddr = payAddr
 		payIntent.PaymentRequest = []byte(rpcPayReq.PaymentRequest)
+		payIntent.Metadata = payReq.Metadata
 	} else {
 		// Otherwise, If the payment request field was not specified
 		// (and a custom route wasn't specified), construct the payment