Add mempool count line to graph

frontend/src/app/interfaces/node-api.interface.ts

@@ -2,6 +2,7 @@ import { Block, Transaction } from "./electrs.interface";
 
 export interface OptimizedMempoolStats {
   added: number;
+  count: number;
   vbytes_per_second: number;
   total_fee: number;
   mempool_byte_weight: number;