private void closeOpenIterators() {
        final HashSet<KeyValueIterator<Bytes, byte[]>> iterators;
        synchronized (openIterators) {
            iterators = new HashSet<>(openIterators);
        }
        if (iterators.size() != 0) {
            log.warn("Closing {} open iterators for store {}", iterators.size(), name);
            for (final KeyValueIterator<Bytes, byte[]> iterator : iterators) {
                iterator.close();
            }
        }
    }
Return an approximate count of key-value mappings in this store.

<code>RocksDB</code> cannot return an exact entry count without doing a
full scan, so this method relies on the <code>rocksdb.estimate-num-keys</code>
property to get an approximate count. The returned size also includes
a count of dirty keys in the store's in-memory cache, which may lead to some
double-counting of entries and inflate the estimate.

@return an approximate count of key-value mappings in the store.
streams/src/main/java/org/apache/kafka/streams/state/internals/RocksDBStore.java