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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.
There's also Boulder too which supposedly is what Let's Encrypt actually runs. But, I believe you have be running Python or Docker on your Linux server, where SmallStep didn't have that requirement.
NOTE:
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Hence, a higher number means a more popular project.