Monitoring service providers have arisen to meet increasing demands from system administrators. Providing a monitoring service requires high-resolution time series, long-term retention, and high write scalability to a time series database (TSDB). Nevertheless, no research to date has examined proposed TSDBs with consideration of the extensibility of data structures for function additions of a monitoring service. As described herein, we introduce a TSDB architecture for automatically tiering on heterogeneous key-value stores (KVS) that combines with an in-memory KVS and an on-disk KVS. Our proposed architecture, by unbundling the data structure on memory and disk as different KVSs, enables the TSDB extensibility to duplicate writes to new data structures optimized for each use case without reducing writing efficiency or data storage efficiency.