default TTLDefinition getTTL() {
        return null;
    }
Get the TTL of the metrics and records data from the selected storage.

@return null if the storage doesn't support TTL customization. Or return the TTL definition from specific storage
implementation.
oap-server/server-core/src/main/java/org/apache/skywalking/oap/server/core/storage/ttl/StorageTTLStatusQuery.java