Unable to return HTTP 429 if function already running

I've tried all the following settings together, but a second request just gets queued and runs after the first is complete. Whether running in Azure Functions v2.x or locally in Visual Studio with SDK v1.0.23

`[Singleton]`

```
{
"version": "2.0",
"http": {
"routePrefix": "api",
"maxOutstandingRequests": 0,
"maxConcurrentRequests": 1,
"dynamicThrottlesEnabled": false
}
}
```

`WEBSITE_MAX_DYNAMIC_APPLICATION_SCALE_OUT = 1`

_Originally posted by @tbdowns in https://github.com/Azure/azure-functions-host/issues/912#issuecomment-433201612_

Reproduce the behavior in Azure Functions v2.x and locally with Visual Studio SDK v1.0.23 using the shown host.json HTTP limits, [Singleton], and WEBSITE_MAX_DYNAMIC_APPLICATION_SCALE_OUT setting. Trace how a second request is handled while the first runs; done means the second request returns HTTP 429 instead of waiting in the queue.

Written by the indexing model from the issue text.