Only set up exporters if providers are properly set

[qianl15]

This PR fixes the issue where DBOS would throw an exception if enable_otlp is set to True but the logger/tracer providers weren't set. Instead of crashing the entire app, it'll log a warning after this change.

Also fixed an issue where DBOS.trace being None (when OTLP tracer wasn't set) could crash the FastAPI app. It should only set the status code if it can find an active span.

[dosubot]

Documentation Updates

1 document(s) were updated by changes in this PR:

DBOS Python

• DBOS Tracing and Span Management (View Changes)

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