cannot find package “go.opentelemetry.io/otel/api/trace” in any of
cannot find package “go.opentelemetry.io/otel/api/global” in any of
cannot find package “go.opentelemetry.io/otel/api/metric” in any of
…

Solution:
Create a new folder named go.opentelemetry.io for the $GOPATH/src directory, and then download the package.
git clone https://github.com/open-telemetry/opentelemetry-go $GOPATH/src/go.opentelemetry.io/otel
Replace $GOPATH with your $GOPATH directory, the same way under Linux