[Feature Request] Attach info about unresolved commands to WFT completions

[Sushisource]

Is your feature request related to a problem? Please describe.

Right now, there is no way when inspecting workflow history to tell if the workflow is actually waiting on something to be resolved (ex: receive a signal) or is completely stuck and will never be able to make progress.

Beyond that, it is immediately obvious how useful it would be to know things like "Oh, I am WFT heartbeating because I'm waiting on a local activity". Or "This workflow is currently waiting on two signals", etc etc.

Describe the solution you'd like

We should add a new field to workflow task completions which allows us to record this information. Possibly, it makes good sense to just have a generic map field, so that we can record anything SDKs feel like without needing to go back and add a new field each time. However, it does make good sense to make the value we record be a proto so that it can be viewed in the UI.

We should record:

• Number of executing local activities
• Counts of how many of each thing that can be awaited are actively being awaited (signals, activities, timers, children, etc)

This will allow us to understand why a workflow completed a task with no commands. If it's not waiting on anything, then by definition the workflow is now stuck. Detecting this situation and alerting the user would be incredibly useful.

[cretz]

Concur. My specific proto suggestion would be adding needed fields on temporal.api.workflowservice.v1.RespondWorkflowTaskCompletedRequest and adding a new proto temporal.api.workflow.v1.CompletedWorkflowTaskInfo (similar to its sibling PendingWorkflowTaskInfo) that is present on a new last_completed_workflow_task field of temporal.api.workflowservice.v1.DescribeWorkflowExecutionResponse. We wouldn't want this in history for obvious reasons.