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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.
It appears that Google wrote some utilities to do this without manually having to deal with HTTP, but they are only available in Java, Python, Node.js, PHP and .NET - not C++. So you will have to write your own C++ library that sends HTTP requests to the web API and handles the received messages and data. You will need some library to handle network sockets and HTTP requests, such as libcurl or Boost Beast (or see under Communication here: https://en.cppreference.com/w/cpp/links/libs)
NOTE:
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Hence, a higher number means a more popular project.