Changes 64-bit traces to left-pad 0s

Doing so helps remove confusion with 64-bit trace ids, which are encoded
by Zipkin with 16 characters (implicitly padded left with zeros).

translation/src/main/java/com/google/cloud/trace/zipkin/translation/TraceTranslator.java

@@ -92,12 +92,8 @@ private void finishTrace(Trace.Builder traceBuilder, Collection<Trace> converted
   static String convertTraceId(Span zipkinSpan) {
     // Stackdriver trace ID's are 128 bits = 16 bytes * 8
     ByteBuffer idBuffer = ByteBuffer.allocate(16); // or 32 characters
-    // Some users call the Stackdriver API and order results by traceId to get a "random sample" of
-    // traces. Duplicating the 64-bit ID instead of padding the ID with zeros will remove some
-    // sampling bias for any projects that contain both Zipkin and Stackdriver traces. Regardless of
-    // if we pad-left 0s or duplicate traceId, this resulting ID won't match Zipkin anyway as it
-    // conditionally encodes 16 characters if high bits are unset (as opposed to padding to 32).
-    idBuffer.putLong(zipkinSpan.traceIdHigh == 0L ? zipkinSpan.traceId : zipkinSpan.traceIdHigh);
+    // Note that when 64-bit trace IDs are used, the left-most 16 characters will be zero
+    idBuffer.putLong(zipkinSpan.traceIdHigh);
     idBuffer.putLong(zipkinSpan.traceId);
     StringBuilder idBuilder = new StringBuilder();
     for (byte b : idBuffer.array()) {

translation/src/test/java/com/google/cloud/trace/zipkin/translation/TraceTranslatorTest.java

@@ -51,19 +51,14 @@ public void testTranslateTrace_128bitInputTraceId() {
         .containsExactly("00000000000000020000000000000001");
   }
 
-  /**
-   * Duplicating the lower 64 bits to construct a 128-bit trace ID removes bias when users sort on
-   * trace ID to choose a random traces. If instead we left-padded zeros, 64-bit traces would always
-   * appear first.
-   */
   @Test
-  public void testTranslateTrace_64BitTraceIdCopiedTwiceToMake128bits() {
+  public void testTranslateTrace_64BitTraceIdLeftPadsZeros() {
     Span span = Span.builder().id(1).traceId(1).name("/a").build();
     TraceTranslator translator = new TraceTranslator("test-project");
 
     assertThat(translator.translateSpans(Arrays.asList(span)))
         .extracting("traceId")
-        .containsExactly("00000000000000010000000000000001");
+        .containsExactly("00000000000000000000000000000001");
   }
 
   @Test
@@ -101,8 +96,8 @@ public void testTranslateTrace() {
         trace1.getTraceId(), trace1,
         trace2.getTraceId(), trace2
     );
-    String key1 = "00000000000000010000000000000001";
-    String key2 = "00000000000000020000000000000002";
+    String key1 = "00000000000000000000000000000001";
+    String key2 = "00000000000000000000000000000002";
     assertTrue(traceMap.containsKey(key1));
     assertTrue(traceMap.containsKey(key2));
     assertEquals(2, traceMap.get(key1).getSpansCount());