samples: PITR samples backup fix (#247)

* samples: creates a backup using the current time

Instead of using the earliest version time of the database, uses the
current time (from spanner). If we used the earliest version time of the
database instead we would be creating an empty backup, since the
database we are backing up is not 1 hour old (default version retention
period).

* samples: addresses pr comments

Refactors create backup sample

Author: thiagotnunes

samples/samples/backup_sample.py

@@ -32,9 +32,14 @@ def create_backup(instance_id, database_id, backup_id):
     instance = spanner_client.instance(instance_id)
     database = instance.database(database_id)
 
+    # Sets the version time as the current server time
+    version_time = None
+    with database.snapshot() as snapshot:
+        results = snapshot.execute_sql("SELECT CURRENT_TIMESTAMP()")
+        version_time = list(results)[0][0]
+
     # Create a backup
     expire_time = datetime.utcnow() + timedelta(days=14)
-    version_time = database.earliest_version_time
     backup = instance.backup(backup_id, database=database, expire_time=expire_time, version_time=version_time)
     operation = backup.create()