Custom cell metadata isn't saved to disk.

Environment data

Version: 1.56.0-insider (user setup) Commit: 845efe5d8e31079cc858131c95c36103d9439e74 Date: 2021-04-09T09:50:06.816Z Electron: 11.4.2 Chrome: 87.0.4280.141 Node.js: 12.18.3 V8: 8.7.220.31-electron.0 OS: Windows_NT x64 10.0.19042

Jupyter extension version: v2021.6.732501275

Expected behaviour

1. Install the .NET Interactive extension.
2. Create a new notebook and select the .NET Interactive kernel and ensure the cell’s language is “C# (.NET Interactive)”
3. Save the notebook.
4. Open in notepad.

Expected: Cell metadata should look like

"metadata": {
  "dotnet_interactive": {
    "language": "csharp"
  }
}

Actual behaviour

Cell metadata is empty. We’re updating the metadata here that makes the object look like:

"metadata": {
  "custom": {
    "dotnet_interactive": {
      "language": "csharp"
    }
  }
}

But it’s not getting persisted to disk.

[colombod]

This is a big issue as every time a notebook is open the user needs to reselect all languages correctly. It is impacting a lot our user experience, @DonJayamanne do you have any sense of the work plan for this?

[colombod]

Yes

What should we do in the meanwhile? This is affecting all our users pretty significantly, do we need to point them at alternative file formats?