Combine per-document stores into one source keyed by each supplied key.
For WorkflowObserver.observeDocs, use the requested document's complete
globalDocumentId as its key. Resolved missing documents remain in the map
with a null value. The snapshot stays stable until an input changes.
Combine per-document stores into one source keyed by each supplied
key. For WorkflowObserver.observeDocs, use the requested document's completeglobalDocumentIdas its key. Resolved missing documents remain in the map with anullvalue. The snapshot stays stable until an input changes.