Increase the buffer size for the trace export batch. (#1911) (#1912)

(cherry picked from commit 9c14ba2)

Author: songy23

impl_core/src/main/java/io/opencensus/implcore/trace/export/ExportComponentImpl.java

@@ -24,7 +24,7 @@
 
 /** Implementation of the {@link ExportComponent}. */
 public final class ExportComponentImpl extends ExportComponent {
-  private static final int EXPORTER_BUFFER_SIZE = 32;
+  private static final int EXPORTER_BUFFER_SIZE = 512;
   // Enforces that trace export exports data at least once every 5 seconds.
   private static final Duration EXPORTER_SCHEDULE_DELAY = Duration.create(5, 0);