Document Protobuf oneof support in Kafka connector

Author: hashhar

docs/src/main/sphinx/connector/kafka.rst

@@ -418,6 +418,7 @@ table description supplier are:
 * New tables can be defined without a cluster restart.
 * Schema updates are detected automatically.
 * There is no need to define tables manually.
+* Some Protobuf specific types like ``oneof`` are supported and mapped to JSON.
 
 Set ``kafka.table-description-supplier`` to ``CONFLUENT`` to use the
 schema registry. You must also configure the additional properties in the following table:
@@ -479,6 +480,45 @@ used to resolve the subject name via the topic name. Note that a case
 insensitive match must be done, as identifiers cannot contain upper case
 characters.
 
+Protobuf-specific type handling in Confluent table description supplier
+"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
+
+When using the Confluent table description supplier, the following Protobuf
+specific types are supported in addition to the :ref:`normally supported types
+<kafka-protobuf-decoding>`:
+
+oneof
++++++
+
+Protobuf schemas containing ``oneof`` fields are mapped to a ``JSON`` field in
+Trino.
+
+For example, given the following Protobuf schema:
+
+.. code-block:: text
+
+    syntax = "proto3";
+
+    message schema {
+        oneof test_oneof_column {
+            string string_column = 1;
+            uint32 integer_column = 2;
+            uint64 long_column = 3;
+            double double_column = 4;
+            float float_column = 5;
+            bool boolean_column = 6;
+        }
+    }
+
+The corresponding Trino row is a ``JSON`` field ``test_oneof_column``
+containing a JSON object with a single key. The value of the key matches
+the name of the ``oneof`` type that is present.
+
+In the above example, if the Protobuf message has the
+``test_oneof_column`` containing ``string_column`` set to a value ``Trino``
+then the corresponding Trino row includes a column named
+``test_oneof_column`` with the value ``JSON '{"string_column": "Trino"}'``.
+
 .. _kafka-sql-inserts:
 
 Kafka inserts
@@ -1350,6 +1390,8 @@ The schema evolution behavior is as follows:
   If the type coercion is supported by Avro, then the conversion happens. An
   error is thrown for incompatible types.
 
+.. _kafka-protobuf-decoding:
+
 Protobuf decoder
 """"""""""""""""