storage/engine: add BenchmarkRocksDBDeleteRangeIterate

Demonstrate the performance problem with iterating through a range
tombstone.

name                                        time/op
RocksDBDeleteRangeIterate/entries=10-8      6.09µs ± 3%
RocksDBDeleteRangeIterate/entries=1000-8     131µs ± 3%
RocksDBDeleteRangeIterate/entries=100000-8  12.3ms ± 3%

See cockroachdb#24029

Release note: None

pkg/storage/engine/rocksdb_test.go

@@ -1390,3 +1390,73 @@ func TestRocksDBDeleteRangeCompaction(t *testing.T) {
 6: "b000000000" - "b000009999"
 `)
 }
+
+func BenchmarkRocksDBDeleteRangeIterate(b *testing.B) {
+	for _, entries := range []int{10, 1000, 100000} {
+		b.Run(fmt.Sprintf("entries=%d", entries), func(b *testing.B) {
+			db := setupMVCCInMemRocksDB(b, "unused").(InMem)
+			defer db.Close()
+
+			makeKey := func(i int) roachpb.Key {
+				return roachpb.Key(fmt.Sprintf("%09d", i))
+			}
+
+			// Create an SST with N entries and ingest it. This is a fast way to get a
+			// lof of entries into RocksDB.
+			{
+				sst, err := MakeRocksDBSstFileWriter()
+				if err != nil {
+					b.Fatal(sst)
+				}
+				defer sst.Close()
+
+				for i := 0; i < entries; i++ {
+					kv := MVCCKeyValue{
+						Key: MVCCKey{
+							Key: makeKey(i),
+						},
+					}
+					if err := sst.Add(kv); err != nil {
+						b.Fatal(err)
+					}
+				}
+
+				sstContents, err := sst.Finish()
+				if err != nil {
+					b.Fatal(err)
+				}
+
+				filename := fmt.Sprintf("ingest")
+				if err := db.WriteFile(filename, sstContents); err != nil {
+					b.Fatal(err)
+				}
+
+				err := db.IngestExternalFiles(context.Background(), []string{filename}, true)
+				if err != nil {
+					b.Fatal(err)
+				}
+			}
+
+			// Create a range tombstone that overlaps all of these entries.
+			from := makeKey(0)
+			to := makeKey(entries)
+			if err := db.ClearRange(MakeMVCCMetadataKey(from), MakeMVCCMetadataKey(to)); err != nil {
+				b.Fatal(err)
+			}
+
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				iter := db.NewIterator(IterOptions{UpperBound: roachpb.KeyMax})
+				iter.Seek(MakeMVCCMetadataKey(from))
+				ok, err := iter.Valid()
+				if err != nil {
+					b.Fatal(err)
+				}
+				if ok {
+					b.Fatal("unexpected key found")
+				}
+				iter.Close()
+			}
+		})
+	}
+}