[ENH]: Add spann metrics

Author: sanketkedia

rust/blockstore/src/arrow/flusher.rs

@@ -75,4 +75,8 @@ impl ArrowBlockfileFlusher {
     pub(crate) fn count(&self) -> u64 {
         self.count
     }
+
+    pub(crate) fn num_blocks(&self) -> usize {
+        self.blocks.len()
+    }
 }

rust/blockstore/src/types/flusher.rs

@@ -38,4 +38,11 @@ impl BlockfileFlusher {
             BlockfileFlusher::ArrowBlockfileFlusher(flusher) => flusher.count(),
         }
     }
+
+    pub fn num_blocks(&self) -> usize {
+        match self {
+            BlockfileFlusher::MemoryBlockfileFlusher(_) => unimplemented!(),
+            BlockfileFlusher::ArrowBlockfileFlusher(flusher) => flusher.num_blocks(),
+        }
+    }
 }