[BUG] Insights standard web test (webTest) frequency is unclearly limited to 300, 600, 900

### API Spec link

https://github.com/Azure/azure-rest-api-specs/blob/0f7fd0892e5b6dbbeb2a49c402ffd690ad5314d5/specification/applicationinsights/resource-manager/Microsoft.Insights/stable/2022-06-15/webTests_API.json#L411-L416

### API Spec version

2022-06-15

### Describe the bug

The frequency of web tests is specified as an integer, but in reality it seems to only allow the following values: 300, 600, 900 (5, 10, 15 minutes respectively). Anything else gives the following error during an ARM deployment:
```
BadRequest: "Value cannot be null. Parameter name: format"
```

Additionally I would also like to report to the API team itself (as bug or feature request), but I am not sure where I can do that.

### Expected behavior

Either any integer is allowed, or integer values are limited to allowed integers documented in the spec.

### Actual behavior

The spec says any integer is allowed, but in reality it seems only 3 specific values are allowed.

### Reproduction Steps

1. Make ARM template for webtest.
2. Set frequency to any other value than 300, 600, 900.
3. Deploy.
4. Experience an error.

### Environment

Host: Windows 10 64-bit
```
> az -v
azure-cli 2.60.0 *
core 2.60.0 *
```

Start with specification/applicationinsights/resource-manager/Microsoft.Insights/stable/2022-06-15/webTests_API.json at lines 411-416 and review the frequency definition. Reproduce the ARM deployment using values other than 300, 600, and 900, then compare the observed service behavior with the API specification. Done means the specification accurately documents the accepted frequency values or the allowed integer range.

Written by the indexing model from the issue text.