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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.
A lot is covered here and here, sar might be useful, getfacl and tcpdump and probably hundred more tools I am not even aware of. The easiest to learn those is to have a problem and identify and solve it with those tools, but to do that you need to know roughly what those tools can do.
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.