BigQuery Storage Write API: JsonStreamWriter cannot transform BigDecimal with 9+ decimals to BigNumeric

[roy-tc]

Environment details

1. OS type and version: OSX 11.6.3
2. Java version: OpenSDK 11
3. version(s): v2.28.3 and later

Steps to reproduce

1. Define a TableSchema with a field of type BIGNUMERIC
2. Serialize a JSON message to Proto containing more than 9 decimals, eg. the number 24.6789012345

Code example

This unit test could be added to JsonToProtoMessageTest to reproduce it:

@Test
  public void testBigDecimalToBigNumericConversion() {
    TableSchema ts =
        TableSchema.newBuilder()
            .addFields(
                0,
                TableFieldSchema.newBuilder()
                    .setName("bignumeric")
                    .setType(TableFieldSchema.Type.BIGNUMERIC)
                    .setMode(TableFieldSchema.Mode.REPEATED)
                    .build())
            .build();
    TestBignumeric expectedProto =
        TestBignumeric.newBuilder()
            .addBignumeric(
                BigDecimalByteStringEncoder.encodeToBigNumericByteString(
                    new BigDecimal("24.6789012345")))
            .build();
    JSONObject json = new JSONObject();
    json.put("bignumeric", Collections.singletonList(new BigDecimal("24.6789012345")));
    DynamicMessage protoMsg =
        JsonToProtoMessage.convertJsonToProtoMessage(TestBignumeric.getDescriptor(), ts, json);
    assertEquals(expectedProto, protoMsg);
  }

Stack trace

[ERROR] com.google.cloud.bigquery.storage.v1.JsonToProtoMessageTest.testBigDecimalToBigNumericConversion  Time elapsed: 0.007 s  <<< ERROR!
com.google.cloud.bigquery.storage.v1.Exceptions$FieldParseError: ByteString scale cannot exceed 9: 24.6789012345
	at com.google.cloud.bigquery.storage.v1.JsonToProtoMessage.convertJsonToProtoMessageImpl(JsonToProtoMessage.java:247)
	at com.google.cloud.bigquery.storage.v1.JsonToProtoMessage.convertJsonToProtoMessage(JsonToProtoMessage.java:139)
	at com.google.cloud.bigquery.storage.v1.JsonToProtoMessageTest.testBigDecimalToBigNumericConversion(JsonToProtoMessageTest.java:1437)
...

Any additional information below

On the following lines, encodeToBigNumericByteString should be called instead of encodeToNumericByteString:

• https://github.com/googleapis/java-bigquerystorage/blob/v2.28.3/google-cloud-bigquerystorage/src/main/java/com/google/cloud/bigquery/storage/v1/JsonToProtoMessage.java#L358
• https://github.com/googleapis/java-bigquerystorage/blob/v2.28.3/google-cloud-bigquerystorage/src/main/java/com/google/cloud/bigquery/storage/v1/JsonToProtoMessage.java#L606

If you agree on this proposed approach, we can create PR with the following fix:
main...tradecloud:java-bigquerystorage:TC-8710-serialize-bigdecimal