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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
Self-hosting on a raspberry pi is relatively affordable and easy, and avoids having to pay a recurring subscription to anyone (though raspberry pis are in short supply right now). Here's a guide I wrote up.
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.