To submit traces to Stackdriver Trace, use Stackdriver Trace client libraries.
In the App Engine standard environment, Java 8, Python 2, and PHP 5 applications don't need to use the Stackdriver Trace client libraries. These runtimes automatically send latency data to Stackdriver Trace for requests to application URIs.
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.