tests: increase timeout to prevent flakiness (googleapis#208)

Author: olavloite

google-cloud-spanner/src/test/java/com/google/cloud/spanner/SessionPoolTest.java

@@ -1691,7 +1691,7 @@ public void testSessionMetrics() throws Exception {
             .setMinSessions(1)
             .setMaxSessions(2)
             .setMaxIdleSessions(0)
-            .setInitialWaitForSessionTimeoutMillis(20L)
+            .setInitialWaitForSessionTimeoutMillis(50L)
             .build();
     FakeClock clock = new FakeClock();
     clock.currentTimeMillis = System.currentTimeMillis();