Depending on what your reporting needs are I would definitely recommend TimescaleDB. I love Prometheus for storing server resource metrics or basic app metrics (counts/timers). For more business-facing data, having the query functionality and visualization tooling of Postgres is a major win. I am working on a similar use-case to you, we considered Cassandra, Timescale, and Clickhouse. With our insert rates and reporting needs, We decided to start working with TimescaleDB.
Can this be used side by side on normal Postgres cluster? As in could I have one DB for app data, and one for metrics data? Considering switching from MySQL (ndb cluster) to running a Postgres cluster and this could be a good motivator.