ZipkinReporter: add trace logging for batching and encoded spans

Motivation:

In order to trace batching, trace level logging will be helpful.

Modifications:

- Log every accumulation;
- Log every received span/list after encoding;

Result:

Easier to troubleshoot if there are any issues.

servicetalk-opentracing-zipkin-publisher/src/main/java/io/servicetalk/opentracing/zipkin/publisher/reporter/HttpReporter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2019, 2021 Apple Inc. and the ServiceTalk project authors
+ * Copyright © 2019-2023 Apple Inc. and the ServiceTalk project authors
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -127,7 +127,12 @@ private PublisherSource.Processor<Span, Span> initReporter(final Builder builder
         if (!builder.batchingEnabled) {
             buffer = newPublisherProcessorDropHeadOnOverflow(builder.maxConcurrentReports);
             spans = fromSource(buffer)
-                    .map(span -> allocator.wrap(spanEncoder.encodeList(Collections.singletonList(span))));
+                    .map(span -> {
+                        // Always encode spans as list: https://github.com/apple/servicetalk/pull/2092
+                        final byte[] bytes = spanEncoder.encodeList(Collections.singletonList(span));
+                        LOGGER.trace("Encoded received span: {}, result={} bytes", span, bytes.length);
+                        return allocator.wrap(bytes);
+                    });
         } else {
             // As we send maxConcurrentReports number of parallel requests, each with roughly batchSizeHint number of
             // spans, we hold a maximum of that many Spans in-memory that we can send in parallel to the collector.
@@ -136,7 +141,12 @@ private PublisherSource.Processor<Span, Span> initReporter(final Builder builder
                     .buffer(forCountOrTime(builder.batchSizeHint, builder.maxBatchDuration,
                             () -> new ListAccumulator(builder.batchSizeHint), client.executionContext().executor()))
                     .filter(accumulate -> !accumulate.isEmpty())
-                    .map(bufferedSpans -> allocator.wrap(spanEncoder.encodeList(bufferedSpans)));
+                    .map(bufferedSpans -> {
+                        final byte[] bytes = spanEncoder.encodeList(bufferedSpans);
+                        LOGGER.trace("Encoded received list of spans (size={}): {}, result={} bytes",
+                                bufferedSpans.size(), bufferedSpans, bytes.length);
+                        return allocator.wrap(bytes);
+                    });
         }
 
         final CompletableSource.Processor spansTerminated = newCompletableProcessor();
@@ -287,6 +297,7 @@ private static final class ListAccumulator implements Accumulator<Span, List<Spa
 
         @Override
         public void accumulate(@Nonnull final Span item) {
+            LOGGER.trace("Accumulating received span: {}", item);
             accumulate.add(requireNonNull(item));
         }
 

servicetalk-opentracing-zipkin-publisher/src/main/java/io/servicetalk/opentracing/zipkin/publisher/reporter/UdpReporter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2019 Apple Inc. and the ServiceTalk project authors
+ * Copyright © 2019-2023 Apple Inc. and the ServiceTalk project authors
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -197,6 +197,7 @@ protected void initChannel(final Channel ch) {
                             public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) {
                                 if (msg instanceof Span) {
                                     byte[] bytes = codec.spanBytesEncoder().encode((Span) msg);
+                                    LOGGER.trace("Encoded received span: {}, result={} bytes", msg, bytes.length);
                                     ByteBuf buf = ctx.alloc().buffer(bytes.length).writeBytes(bytes);
                                     ctx.write(new DatagramPacket(buf, (InetSocketAddress) collectorAddress), promise);
                                 } else {