fix: Set batching settings on SinglePartitionPublisher

[dpcollins-google]

Writing to a Lite topic will work with this fix.
But a pipeline that writes a bounded source to a Lite topic doesn't exit.

[codecov]

Codecov Report

Merging #563 (00c6444) into master (343afbf) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master     #563      +/-   ##
============================================
- Coverage     72.63%   72.62%   -0.02%     
  Complexity      861      861              
============================================
  Files           156      156              
  Lines          4488     4489       +1     
  Branches        211      211              
============================================
  Hits           3260     3260              
- Misses         1107     1108       +1     
  Partials        121      121              

Impacted Files	Coverage Δ	Complexity Δ
...a/com/google/cloud/pubsublite/beam/Publishers.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)

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[anguillanneuf]

Tested with a simple example like below and it worked, but the pipeline doesn't exit with a bounded source. @dpcollins-google is aware of it and look into it a bit later.

import com.google.cloud.pubsublite.Message;
import com.google.cloud.pubsublite.proto.PubSubMessage;
// ...
    pipeline
        .apply(Create.of("Hello!", "Hello again"))
        .apply(ParDo.of(new DoFn<String, PubSubMessage>() {
          @ProcessElement
          public void processElement(ProcessContext context) {
            ImmutableListMultimap<String, ByteString> multimap =
                ImmutableListMultimap.of("Year", ByteString.copyFromUtf8("2021"));
            Message message = Message.builder()
                .setData(ByteString.copyFromUtf8(context.element()))
                .setAttributes(multimap)
                //.setEventTime(fromMillis(System.currentTimeMillis()))
                .build();
            LOG.info(Instant.now() + "\t" + message);
            context.output(message.toProto());
          }
        }))
        .apply("Write to Pub/Sub Lite", PubsubLiteIO.write(publisherOptions));