feat: release as GA

[busunkim96]

Also removes mentions of "Stackdriver" to match https://cloud.google.com/trace/docs/reference

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Package name: google-cloud-trace
Current release: alpha
Proposed release: GA

Instructions

Check the lists below, adding tests / documentation as required. Once all the "required" boxes are ticked, please create a release and close this issue.

Required

• 28 days elapsed since last beta release with new API surface (last release September 14 https://github.com/googleapis/python-trace/releases/tag/v1.0.0)
• Server API is GA (v1 and v2)
• Package API is stable, and we can commit to backward compatibility
• All dependencies are GA

Optional

• Most common / important scenarios have descriptive samples
• Public manual methods have at least one usage sample each (excluding overloads)
• Per-API README includes a full description of the API
• Per-API README contains at least one “getting started” sample using the most common API scenario
• Manual code has been reviewed by API producer
• Manual code has been reviewed by a DPE responsible for samples
• 'Client Libraries' page is added to the product documentation in 'APIs & Reference' section of the product's documentation on Cloud Site