As fas as I have seen, 99% of in-house solutions are tailored for exactly 1 use case - the company that built it. Existing solutions like InfluxDB are made way more generic.

Companies like Twitter, Uber etc can spend many millions and won't notice so they can build tailored databases for themselves.

Will someone else use it? Probably not (sure unless this is something new or that differs a lot from existing solutions and gives a lot of benefits)

Also, TimescaleDB is a general purpose time-series database, which means that it not only works for metrics data, but also many other datatypes (strings, arrays, JSON, etc). You can also create indexes on any of these fields.

Because TimescaleDB is built on Postgres, it is also a relational time-series database. Ie you can store metadata / business data alongside your time-series data.

(Note: I work on TimescaleDB.)

MetricsDB appears to be HDFS-based. TimescaleDB is Postgres-based and InfluxDB is memory and log-based. Prometheus is file-based.