Add JsonElement.GetPropertyCount()

[WeihanLi]

fixes #104692

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[eiriktsarpalis]

This looks great. Can I mark this ready for review or do you still have changes you want to make?

[WeihanLi]

@eiriktsarpalis I still have some issues with the result of GetPropertyCount
for example, the use case from the issue, JsonSerializer.Deserialize<JsonElement>("""{ "foo" : 1, "bar" : 2 }""").GetPropertyCount() returns 5, does not return 2 as expected, think maybe need some filter from the current filter, do you have any idea about this?

[eiriktsarpalis]

Oh, interesting. This suggests that there might be an issue with the value of the SizeOrLength property in the context of objects:

runtime/src/libraries/System.Text.Json/src/System/Text/Json/Document/JsonDocument.cs

Lines 143 to 152 in bcca12c

	internal int GetPropertyCount(int index)
	{
	CheckNotDisposed();
	DbRow row = _parsedData.Get(index);
	CheckExpectedType(JsonTokenType.StartObject, row.TokenType);
	return row.SizeOrLength;
	}

It might be worth investigating what the value represents in objects and see if there's an easy way to obtain the actual property count from it.

[eiriktsarpalis]

The issue with this approach is that needs to perform a traversal of the entire object to compute the size. This is already possible with EnumerateObject(), we should only include this API if we can guarantee that it's constant-time.

Were you able to determine what the row.SizeOrLength value represents in the case of objects and if so, can be meaningfully used (or changed) so that we can obtain the property count?

[WeihanLi]

Some test cases failed when I tried to update the SizeOrLength for object properties count, so I tried this way.
Would try to update the SizeOrLength to do some research on the failed cases, thanks for the input

[WeihanLi]

closed in favor of #106503