feat(db): add size api for db

Former-commit-id: 9d20366e22996fc7bb3e7e983ebfa9999591ba6b

cpp/src/db/DB.h

@@ -39,6 +39,8 @@ public:
     virtual Status search(const std::string& group_id, size_t k, size_t nq,
             const float* vectors, const meta::DatesT& dates, QueryResults& results) = 0;
 
+    virtual Status size(long& result) = 0;
+
     virtual Status drop_all() = 0;
 
     virtual Status count(const std::string& group_id, long& result) = 0;

cpp/src/db/DBImpl.cpp

@@ -427,6 +427,11 @@ Status DBImpl<EngineT>::count(const std::string& group_id, long& result) {
     return _pMeta->count(group_id, result);
 }
 
+template<typename EngineT>
+Status DBImpl<EngineT>::size(long& result) {
+    return  _pMeta->size(result);
+}
+
 template<typename EngineT>
 DBImpl<EngineT>::~DBImpl() {
     {

cpp/src/db/DBImpl.h

@@ -55,6 +55,8 @@ public:
 
     virtual Status count(const std::string& group_id, long& result) override;
 
+    virtual Status size(long& result) override;
+
     virtual ~DBImpl();
 
 private: