lnrpc: add htlc attempt id

lnrpc/routerrpc/router.swagger.json

@@ -524,6 +524,11 @@
     "lnrpcHTLCAttempt": {
       "type": "object",
       "properties": {
+        "attempt_id": {
+          "type": "string",
+          "format": "uint64",
+          "description": "The unique ID that is used for this attempt."
+        },
         "status": {
           "$ref": "#/definitions/HTLCAttemptHTLCStatus",
           "description": "The status of the HTLC."

lnrpc/routerrpc/router_backend.go

@@ -861,6 +861,7 @@ func (r *RouterBackend) MarshalHTLCAttempt(
 	}
 
 	rpcAttempt := &lnrpc.HTLCAttempt{
+		AttemptId:     htlc.AttemptID,
 		AttemptTimeNs: MarshalTimeNano(htlc.AttemptTime),
 		Route:         route,
 	}