Bump spanner stale read from 10 to 15 seconds. (#915)

At the request of the spanner team.

Author: SurferJeffAtGoogle

spanner/cloud-client/src/main/java/com/example/spanner/SpannerSample.java

@@ -408,7 +408,7 @@ static void readOnlyTransaction(DatabaseClient dbClient) {
   static void readStaleData(DatabaseClient dbClient) {
     ResultSet resultSet =
         dbClient
-            .singleUse(TimestampBound.ofExactStaleness(10, TimeUnit.SECONDS))
+            .singleUse(TimestampBound.ofExactStaleness(15, TimeUnit.SECONDS))
             .read("Albums",
                 KeySet.all(),
                 Arrays.asList("SingerId", "AlbumId", "MarketingBudget"));

spanner/cloud-client/src/test/java/com/example/spanner/SpannerSampleIT.java

@@ -86,9 +86,9 @@ public void testSample() throws Exception {
     assertThat(out).contains("1 1 Total Junk");
     runSample("addmarketingbudget");
 
-    // wait for 10 seconds to elapse and then run an update, and query for stale data
+    // wait for 15 seconds to elapse and then run an update, and query for stale data
     lastUpdateDataTimeInMillis = System.currentTimeMillis();
-    while (System.currentTimeMillis() < lastUpdateDataTimeInMillis + 11000) {
+    while (System.currentTimeMillis() < lastUpdateDataTimeInMillis + 16000) {
       Thread.sleep(1000);
     }
     runSample("update");