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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.
Sweden. Their app is open source, stores and protects the covid certificate locally on the device but allows for tracking as well if it's permitted in the OS. It also doesn't have Google services framework.
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.