Protobuf message with oneof generates too verbose document

Hi there,
having a Protobuf schema like the following generates too verbose documents in OpenSearch

```
syntax = "proto3";
option java_package = "com.test";
option java_multiple_files = true;

message Event {
message Value {
oneof value {
int32 int_value = 1;
int64 long_value = 2;
float float_value = 3;
double double_value = 4;
bool boolean_value = 5;
bytes bytes_value = 6;
string string_value = 7;
}
}
map values = 1;
}
```

A message containing a values map with each type type once will produce the following document:
```
"value": {
"doubleVal": {
"value_0": {
"int_value": null,
"long_value": null,
"float_value": null,
"double_value": 1.23,
"boolean_value": null,
"bytes_value": null,
"string_value": null
}
},
"intVal": {
"value_0": {
"int_value": 123,
"long_value": null,
"float_value": null,
"double_value": null,
"boolean_value": null,
"bytes_value": null,
"string_value": null
}
},
"floatVal": {
"value_0": {
"int_value": null,
"long_value": null,
"float_value": 2.34,
"double_value": null,
"boolean_value": null,
"bytes_value": null,
"string_value": null
}
},
"boolVal": {
"value_0": {
"int_value": null,
"long_value": null,
"float_value": null,
"double_value": null,
"boolean_value": true,
"bytes_value": null,
"string_value": null
}
},
"longVal": {
"value_0": {
"unit": null,
"int_value": null,
"long_value": 112233,
"float_value": null,
"double_value": null,
"boolean_value": null,
"bytes_value": null,
"string_value": null
}
}
}
```