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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.
Technically, reddit is sort of open source, or it used to be, the repo hasn't been updated in a long time/discontinued in 2017. It takes a bit to set up, and it's missing a bit of features (they wouldn't put proprietary stuff in it anyways), but ya.
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.