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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.
https://github.com/sensu/sensu-go is pretty good, flexible and scalable. Got around 400 servers with a total of around 10.000 checks.
NOTE:
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Hence, a higher number means a more popular project.