Fix date format changing during UI updates by restoring request execution context

[JamesNK]

Fixes #2554

Switching between date formats bug:

• Started when request localization was introduced. Blazor request would use the browser culture to render the UI.
• However, UI updates triggered by telemetry subscriptions were run using the gRPC service's execution context. Requests to gRPC don't have a header indicating culture, so they defaulted to the systems.
• The default system culture then flowed in Blazor UI when telemetry subscription fired and called StateHasChanged().

The fix is to capture the execution context when the Blazor page subscribes to telemetry changes, and then restore it when running the subcription. The restored Blazor execution context sets the right culture for the callback to run in.

@davidfowl You've worked with execution contexts the most here. Does this look right to you?

Microsoft Reviewers: Open in CodeFlow

[DamianEdwards]

Shouldn't Blazor's dispatcher be handling this automatically?

[davidfowl]

Shouldn't Blazor's dispatcher be handling this automatically?

No blazor doesn't handle this. This is when you have a callback API, you have to build something like this if you want to capture the ExecutionContext and have things like async locals work (that's also how you create a memory leak 😄 )

[JamesNK]

Shouldn't Blazor's dispatcher be handling this automatically?

I expected it should. I expected InvokeAsync to restore the original HTTP request execution context (or at least the culture) but it doesn't. There are some GH issues from customers in aspnetcore repo where Blazor folks say this is by design and you're reponsible for restoring the context.

[davidfowl]

It captures the sync context, not the execution context.

[JamesNK]

The end result is weirdness around culture though 🤷‍♂️

Anyway, no automatic culture restore is the world we live in, and this change isn't big or complicated so I'm happy with this change to fix the problem.

David, could you approve if the EC related code is good 🙏

[davidfowl]

Anyway, no automatic culture restore is the world we live in, and this change isn't big or complicated so I'm happy with this change to fix the problem.

Dont build callback APIs 😄

[JamesNK]

Would changing the subscription API to have an subscription.WaitForNextUpdateAsync() method be better?

I could do that, but OTEL subscriptions are used in quite a few places. It would be a day of work to change the dashboard everywhere. Do you want that rather than adding execution context to the callback?

[JamesNK]

@davidfowl Changes from offline conversation applied.