I think TimescaleDB works well up to the single-digit terabyte range (according to various sources). If you need a solution in the multi-digit terabyte or petabyte range, then you probably need something like a distributed VictoriaMetrics setup.
Of course! The question is only in your requirements. Keeping a simple counter with limited cardinality should work just great. But nowadays monitoring is much more serious than that. For monitoring k8s clusters the average ingestion rate of metrics per second varies from 100K to 2Mil. I don't know if, resource-wise, it would be a right decision to use Postgres for storing this.
So when requirements are high, and they are for real-time infrastructure and applications monitoring, it is better to consider something like ClickHouse (for people familiar with Postgres) or VictoriaMetrics (for people familiar with Prometheus).
Redis is great. This gist is more about how it's up to you how much you centralize on postgres. But overall being able to offload from the database has value, so "PostgreSQL is Enough" should not read as "You shouldn't need more than PostgreSQL"
Metrics in an unlogged table could be great if you want to query those metrics against existing data. It depends