Stream SIRI-ET initial data HTTP response directly into XML parser

Instead of buffering the entire HTTP response (~325-580 MB) into a
ByteString before parsing, stream the response body directly into
SiriXml.parseXml() via StAX. This eliminates a large short-lived
allocation that contributes to GC pressure during startup.

The streaming pattern is already used by SiriAzureUpdater and
SiriFmDataSource.

Author: vpaturet

application/src/main/java/org/opentripplanner/updater/trip/siri/updater/google/GooglePubsubEstimatedTimetableSource.java

@@ -265,16 +265,12 @@ private Optional<ServiceDelivery> serviceDelivery(ByteString data) {
    */
   private void initializeData() {
     if (dataInitializationUrl != null) {
-      LOG.info("Fetching initial data from {}", dataInitializationUrl);
+      LOG.info("Fetching and parsing initial data from {}", dataInitializationUrl);
       final long t1 = System.currentTimeMillis();
-      ByteString value = fetchInitialData();
+      var serviceDelivery = fetchAndParseInitialData();
       final long t2 = System.currentTimeMillis();
-      LOG.info(
-        "Fetching initial data - finished after {} ms, got {}",
-        (t2 - t1),
-        FileSizeToTextConverter.fileSizeToString(value.size())
-      );
-      serviceDelivery(value)
+      LOG.info("Fetching and parsing initial data - finished after {} ms", (t2 - t1));
+      serviceDelivery
         .map(serviceDeliveryConsumer)
         .ifPresent(future -> {
           try {
@@ -299,17 +295,18 @@ private void initializeData() {
   }
 
   /**
-   * Fetch the backlog of messages over HTTP from the configured data initialization URL.
+   * Fetch the backlog of messages over HTTP and parse the XML response.
    */
-  private ByteString fetchInitialData() {
+  private Optional<ServiceDelivery> fetchAndParseInitialData() {
     try (OtpHttpClientFactory otpHttpClientFactory = new OtpHttpClientFactory()) {
       var otpHttpClient = otpHttpClientFactory.create(LOG);
-      return otpHttpClient.getAndMap(
+      var siri = otpHttpClient.getAndMap(
         dataInitializationUrl,
         initialGetDataTimeout,
         HttpHeaders.of(Map.of("Content-Type", "application/xml")),
-        response -> ByteString.readFrom(response.body())
+        response -> SiriXml.parseXml(response.body())
       );
+      return Optional.ofNullable(siri.getServiceDelivery());
     }
   }