Stackdriver Trace is automatically enabled for all applications running on the App Engine standard environment. App Engine standard environment applications don't require any additional instrumentation libraries.
Customers tracing applications on VMs or containers (including the Compute Engine, App Engine flexible environment, Google Kubernetes Engine, and those running with other cloud providers or on-premises) should use client libraries to submit traces to the service.
Across all languages, Google is transitioning to make use of OpenCensus, a set of open source tracing and stats instrumentation libraries that work with multiple backends.
See the setup guidance for your language's client library for more detailed information:
You can also receive traces from Zipkin tracers. For more information, go to Using Stackdriver Trace with Zipkin.