@Test void writeList_offset_startsWithSpanKeyAndLengthPrefix() {
    byte[] bytes = new byte[2048];
    writer.writeList(List.of(CLIENT_SPAN, CLIENT_SPAN), bytes, 0);

    assertThat(bytes)
      .startsWith((byte) 10, SPAN.sizeOfValue(CLIENT_SPAN));
  }
proto messages always need a key, so the non-list form is just a single-field
zipkin/src/test/java/zipkin2/internal/Proto3SpanWriterTest.java