libs/db: Add cleveldb.Stats() (#3379)

Fixes: #3378

* Add stats to cleveldb implementation

* update changelog

* remote TODO

also
- sort keys
- preallocate memory

* fix const initializer []string literal is not a constant

* add test

libs/db/c_level_db.go

@@ -128,10 +128,18 @@ func (db *CLevelDB) Print() {
 
 // Implements DB.
 func (db *CLevelDB) Stats() map[string]string {
-	// TODO: Find the available properties for the C LevelDB implementation
-	keys := []string{}
+	keys := []string{
+		"leveldb.aliveiters",
+		"leveldb.alivesnaps",
+		"leveldb.blockpool",
+		"leveldb.cachedblock",
+		"leveldb.num-files-at-level{n}",
+		"leveldb.openedtables",
+		"leveldb.sstables",
+		"leveldb.stats",
+	}
 
-	stats := make(map[string]string)
+	stats := make(map[string]string, len(keys))
 	for _, key := range keys {
 		str := db.db.PropertyValue(key)
 		stats[key] = str

libs/db/c_level_db_test.go

@@ -99,3 +99,12 @@ func TestCLevelDBBackend(t *testing.T) {
 	_, ok := db.(*CLevelDB)
 	assert.True(t, ok)
 }
+
+func TestCLevelDBStats(t *testing.T) {
+	name := fmt.Sprintf("test_%x", cmn.RandStr(12))
+	dir := os.TempDir()
+	db := NewDB(name, LevelDBBackend, dir)
+	defer cleanupDBDir(dir, name)
+
+	assert.NotEmpty(t, db.Stats())
+}