Add workflow.GetSpanContext and workflow.WithSpanContext API

[shijiesheng]

Detailed Description

• GetSpanContext API is used for fetch span context from workflow if it exists
• WithSpanContext() API is used for overriding the existing span context in the workflow so it could be passed to activities and child workflows

Impact Analysis

• Backward Compatibility: Yes, they are new APIs
• Forward Compatibility: Yes, WithSpanContext would write the existing span context field and it's readable for old client versions

Testing Plan

• Unit Tests: No
• Persistence Tests: No
• Integration Tests: Yes
• Compatibility Tests: No

Rollout Plan

• What is the rollout plan? new release
• Does the order of deployment matter? no
• Is it safe to rollback? Does the order of rollback matter? Safe
• Is there a kill switch to mitigate the impact immediately? Rollback to previous version and remove usages of the new APIs

Why?

Users can now access baggage items in the workflow code.

[taylanisikdemir]

should we also expose contextWithSpan or is there another way for users to add their existing context?

[shijiesheng]

Now I'm a bit concerned about the determinism if users start a new span inside the workflow.

[taylanisikdemir]

I guess we don't have a clear story around replay traces. It wouldn't cause non-determinism though. Just weird/duplicate traces potentially.
Are you suggesting to expose the span via GetSpanContext but don't let user set/override the span in the context?

[shijiesheng]

You are right. It's still deterministics. Replay is going to cause issues. We will see multiple child spans if we allow users to set them.

[shijiesheng]

Using Sideeffect would work. But is span serializable or recoverable?

[taylanisikdemir]

Not sure. We can try and see. It sounds like we need to start a new span even for replays but let's discuss that separately.

[shijiesheng]

SpanContext is serializable and should be passed. With that said, we'll require users to start a short-lived new span just to modify spancontext. Please refer the examples in the comments

[shijiesheng]

I've added an integration test which works.