feat: add capacity api for prealloc (#15)

* feat: add api to get the capacity of prealloc

* fix: doc

---------

Co-authored-by: Breeze-P <noreply@greptime.com>

src/producer/prealloc.rs

@@ -18,12 +18,17 @@ pub struct PreAlloc {
 }
 
 impl PreAlloc {
-    /// Get the slice of the pre-allocated.
+    /// Get the slice of the [`PreAlloc`].
     pub fn slice(&self) -> &[u8] {
         self.data_block.slice().unwrap()
     }
 
-    /// Write data to the pre-allocated.
+    /// Get the capacity of the [`PreAlloc`].
+    pub fn capacity(&self) -> u32 {
+        self.data_block.capacity()
+    }
+
+    /// Write data to the [`PreAlloc`].
     pub fn write(&mut self, data: &[u8]) -> Result<()> {
         self.data_block.write(data)
     }

src/ringbuf/data_block.rs

@@ -147,7 +147,7 @@ impl<T> DataBlock<T> {
         self.capacity() + HEADER_LEN as u32
     }
 
-    fn capacity(&self) -> u32 {
+    pub(crate) fn capacity(&self) -> u32 {
         self.header.capacity()
     }