feat: opt-in tracing for periodic log uploads

[Shriprasad-P]

Fixes

Fixes #3307

Summary

Wraps periodic log upload work in Metaflow's existing traced() helper so uploads emit spans only when tracing is already enabled. Default behavior stays a no-op.

Test plan

• Unit tests for disabled tracing (no-op) and enabled attribute capture
• Existing save_logs_periodically tests still pass

[greptile-apps]

Greptile Summary

Adds opt-in tracing around periodic log uploads while preserving no-op behavior when tracing is disabled.

• Keeps the upload inside the active span so its duration and nested instrumentation are captured.
• Records upload size, changed-file count, elapsed time, return code, success, and exception type.
• Extends the tracing context manager to expose the active span and adds coverage for enabled, disabled, failed, and exceptional uploads.

Confidence Score: 5/5

The PR appears safe to merge; the previously reported span-lifetime issue is fully fixed and no new actionable failures remain.

The upload now executes inside the traced context, completion attributes are attached before the span closes, disabled tracing remains a no-op, and upload exceptions remain non-fatal.

Important Files Changed

Filename	Overview
metaflow/mflog/save_logs_periodically.py	Wraps periodic uploads in an active trace and records outcome attributes without changing their non-fatal behavior.
metaflow/tracing/init.py	Makes the disabled tracing context explicitly yield None for compatibility with span-aware callers.
metaflow/tracing/tracing_modules.py	Exposes the active span from the tracing context manager so callers can add completion attributes.
test/unit/test_save_logs_periodically.py	Covers disabled tracing, span lifetime, successful and failed return codes, and upload exceptions.

_{Reviews (3): Last reviewed commit: "Fix periodic log upload span lifetime" | Re-trigger Greptile}