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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.
In Ruby-on-Rails it's part of the authentication framework Devise https://github.com/tinfoil/devise-two-factor and related modules including creating QR codes, backup codes etc. PHP Laraval has similar libraries https://jetstream.laravel.com/2.x/features/two-factor-authen...
Auth0 and similar commercial companies might very good at marketing. Try replacing "API" with "library" or a programming language name.
> It's just not valuable enough for anyone to create
For a commercial API you need to charge $10 USD/month, or equivalent in terms of per-API-call, otherwise your service won't be profitable. You call 5 US cent already expensive, so I think you're right, it's not valuable enough.