Provide better diagnostics for JS interop

[chucker]

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

I am trying to wrap a JS library (in this case, Leaflet), but the tooling makes mistakes hard and time-consuming to figure out.

For example, consider this JS code:

getSize: function (mapId) {
    return maps[mapId].getSize();
},

And then this C# wrapper around it:

public static async ValueTask<SizeF> GetSize(IJSRuntime jsRuntime, string mapId)
    => jsRuntime.InvokeAsync<SizeF>($"{_BaseObjectContainer}.getSize", mapId);

Similar methods exist, and work fine, but this one will return (when awaited) a SizeF of 0, 0. (Can you guess why? I couldn't — I assumed something was initialized wrong on the JS end.)

I had to add various logging code manually to figure out the issue: Leaflet's getSize() returns a Point, so its properties, from the C# side, would become X and Y, not Width and Height.

Trouble is, Blazor doesn't throw an exception, or log a warning — it just maps nothing, and thus the SizeF has an empty width.

Describe the solution you'd like

The methods on IJSInterop should have logging:

1. there should be Debug (perhaps Trace) logging to always log (even in the presumed success case) how the C# in arguments get serialized to JSON, how the JS result comes back as JSON, and how that gets mapped back to C#
2. there should be Warn logging to log when a mapping from JS back to C# didn't succeed. Due to lack of static typing, the reverse can't be validated, but this direction can: JSInterop knows which properties the generic type TValue has, and it knows if the returned JSON contains a property that the type does not.
3. there should especially be a Warn logging if no properties were mapped even though a JSON object came back. That's rarely got to be the intended result.

Additional context

1 of those would've helped me track down dotnet/runtime#39208 a lot faster.
2/3 would have helped me today.

[mkArtakMSFT]

Thanks for contacting us.
We rely on the System.Text.Json serializer for this, which currently doesn't support logging. Feel free to file a separate issue on the dotnet/runtime repo to add that support. When done, we can enable it on our side too.