Add memory usage tracking method to file prefetch buffer

file/file_prefetch_buffer.h

@@ -128,7 +128,7 @@ struct BufferInfo {
             offset < offset_ + async_req_len_);
   }
 
-  size_t CurrentSize() { return buffer_.CurrentSize(); }
+  size_t CurrentSize() const { return buffer_.CurrentSize(); }
 };
 
 enum class FilePrefetchBufferUsage {
@@ -396,6 +396,19 @@ class FilePrefetchBuffer {
   // Callback function passed to underlying FS in case of asynchronous reads.
   void PrefetchAsyncCallback(FSReadRequest& req, void* cb_arg);
 
+  size_t GetTotalBufferSize() const {
+    size_t total = 0;
+    for (const BufferInfo* buf_info : bufs_) {
+      if (buf_info != nullptr) {
+        total += buf_info->CurrentSize();
+      }
+    }
+    if (overlap_buf_ != nullptr) {
+      total += overlap_buf_->CurrentSize();
+    }
+    return total;
+  }
+
   void TEST_GetBufferOffsetandSize(
       std::vector<std::tuple<uint64_t, size_t, bool>>& buffer_info) {
     for (size_t i = 0; i < bufs_.size(); i++) {