[MetricsAdvisor] Make collections returned by service methods pageables

[kinelski]

Updated methods that return IReadOnlyList to return pageables. Getting this PR out first so the samples PR doesn't get too big.

Fixes #15927.

[maririos]

Me learning, is this the only way to get the list of items after they are in the AsyncPageable type?

[kinelski]

It's the only "natural" approach, as far as I know. AsyncPageable implements the IAsyncEnumerable interface, and that's why it's accessed like this (more info about them here).

I think you could do something like this as well:

var enumerator = adminClient.GetSomethingAsync(...).GetAsyncEnumerator();

while (await enumerator.MoveNextAsync())
{
  var element = enumerator.Current;
  // Do something with <element>
}

That's probably what happens under the hood. I don't see a good reason for preferring this approach, though (maybe if you're using an older version of C# and can't use await foreach).

[maririos]

I remembered I saw this somewhere once in appconfig. You could use ToEnumerableAsync.

[kinelski]

This is only a convenience method in our test framework, though. Good to know.

I thought you were asking from a user's point of view.

[christothes]

Sorry, I forgot about this one!