What's the difference between this and opentracing? AFAIK opentracing is built by the people who created Dapper at Google, and the project also has a set of standards for HTTP transport.
They're different in scope and goals. Census is composed of a context propagation format (which the blog post mentioned), a single distribution of language-specific libraries that include instrumentation hooks for popular web / RPC frameworks and metrics + trace exporters for various backends, and an optional agent that you can run locally to view metrics and RPC stats.