tracing: use opentracing.GlobalTracer instead of globalTracer

[redloaf]

💸 TL;DR

There are some places where globalTracer is referenced instead of the opentracing.GlobalTracer(). If these ever differ (which is entirely possible), this breaks assumptions about how this library integrates with opentracing.

📜 Details

If nil is passed as the tracer to newTrace (which is incredibly common), use opentracing's global tracer instead of baseplate's.

🧪 Testing Steps / Validation

✅ Checks

• [ ] CI tests (if present) are passing
• [ ] Adheres to code style for repo
• [ ] Contributor License Agreement (CLA) completed if not a Reddit employee