Can SQL Server 2016 extract node names from JSON?

Select [key] from default OPENJSON schema.

DECLARE @json_doc nvarchar(4000) = '{"Name1":"Value1", "Name2":"Value2"}';

SELECT [key]
FROM OPENJSON(@json_doc);
GO

| key   |
| :---- |
| Name1 |
| Name2 |

dbfiddle here

To add to this answer.

Get all JSON keys in a column:

SELECT Keys.[key]
FROM dbo.<table_name> t
CROSS APPLY (
    SELECT [key] [key]
    FROM OPENJSON((SELECT t.<column_name>))
) Keys

E.g., Table:

+------------------+
| JSONValues       |
+==================+
| {"myKey":1}      |
+------------------+
| {"myOtherKey":1} |
+------------------+

Query Result:

+------------+
| Result     |
+============+
| myKey      |
+------------+
| myOtherKey |
+------------+

Get all JSON keys where JSON is array of objects:

SELECT DISTINCT Keys.[key]
FROM dbo.<table_name, sysname, sample_table> t
CROSS APPLY (
    SELECT x.[value] [value]
    FROM OPENJSON((SELECT t.<json_colum_name, sysname, sample_column>)) x
) ArrayJSON
CROSS APPLY (
    SELECT [key] [key]
    FROM OPENJSON((SELECT ArrayJSON.[value])) x
) Keys

Table:

+----------------------------+
| JSONValues                 |
+============================+
| [{"myKey":1},{"myKey2":2}] |
+----------------------------+
| [{"myKey3":3}]             |
+----------------------------+

Query Result:

+--------+
| Result |
+========+
| myKey  |
+--------+
| myKey2 |
+--------+
| myKey3 |
+--------+