perf: Add benchmarks for BtreeMap::Iterator::count() (#191)

Co-authored-by: Islam El-Ashi <islam.elashi@dfinity.org>

benchmarks/src/btreemap.rs

@@ -1,3 +1,5 @@
+use std::ops::Bound;
+
 use crate::Random;
 use canbench::{benchmark, macros::bench, BenchResult};
 use ic_stable_structures::{storable::Blob, BTreeMap, DefaultMemoryImpl, Storable};
@@ -219,6 +221,39 @@ pub fn btreemap_insert_10mib_values() -> BenchResult {
     })
 }
 
+#[bench]
+pub fn btreemap_iter_count_small_values() -> BenchResult {
+    let mut btree = BTreeMap::new(DefaultMemoryImpl::default());
+    let size: u32 = 10_000;
+    for i in 0..size {
+        btree.insert(i, vec![]);
+    }
+
+    benchmark(|| {
+        btree
+            .range((Bound::Included(0), Bound::Included(size)))
+            .count();
+    })
+}
+
+#[bench]
+pub fn btreemap_iter_count_10mib_values() -> BenchResult {
+    let mut btree = BTreeMap::new(DefaultMemoryImpl::default());
+
+    let size: u8 = 200;
+
+    // Insert 200 10MiB values.
+    for i in 0..size {
+        btree.insert(i, vec![0u8; 10 * 1024]);
+    }
+
+    benchmark(|| {
+        btree
+            .range((Bound::Included(0), Bound::Included(size)))
+            .count();
+    })
+}
+
 /// Benchmarks removing keys from a BTreeMap.
 #[bench]
 pub fn btreemap_remove_blob_4_1024() -> BenchResult {