feat(spanner): A new field snapshot_timestamp is added to message .google.spanner.v1.CommitResponse (#2350)

* feat(spanner): A new field `snapshot_timestamp` is added to message `.google.spanner.v1.CommitResponse`
docs(spanner): A comment for field `commit_stats` in message `.google.spanner.v1.CommitResponse` is changed
docs(spanner): A comment for field `precommit_token` in message `.google.spanner.v1.CommitResponse` is changed
docs(spanner): A comment for message `.google.spanner.v1.TransactionOptions` is changed
docs(spanner): A comment for enum value `READ_LOCK_MODE_UNSPECIFIED` in enum `ReadLockMode` is changed
docs(spanner): A comment for enum value `PESSIMISTIC` in enum `ReadLockMode` is changed
docs(spanner): A comment for enum value `OPTIMISTIC` in enum `ReadLockMode` is changed
docs(spanner): A comment for field `multiplexed_session_previous_transaction_id` in message `.google.spanner.v1.TransactionOptions` is changed
docs(spanner): A comment for field `exclude_txn_from_change_streams` in message `.google.spanner.v1.TransactionOptions` is changed
docs(spanner): A comment for message `.google.spanner.v1.MultiplexedSessionPrecommitToken` is changed

PiperOrigin-RevId: 784456709

Source-Link: googleapis/googleapis@62babf2

Source-Link: googleapis/googleapis-gen@c1d5fdd
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Author: gcf-owl-bot[bot]

protos/google/spanner/v1/commit_response.proto

@@ -44,16 +44,21 @@ message CommitResponse {
   // The Cloud Spanner timestamp at which the transaction committed.
   google.protobuf.Timestamp commit_timestamp = 1;
 
-  // The statistics about this Commit. Not returned by default.
+  // The statistics about this `Commit`. Not returned by default.
   // For more information, see
   // [CommitRequest.return_commit_stats][google.spanner.v1.CommitRequest.return_commit_stats].
   CommitStats commit_stats = 2;
 
-  // Clients should examine and retry the commit if any of the following
-  // reasons are populated.
+  // You must examine and retry the commit if the following is populated.
   oneof MultiplexedSessionRetry {
     // If specified, transaction has not committed yet.
-    // Clients must retry the commit with the new precommit token.
+    // You must retry the commit with the new precommit token.
     MultiplexedSessionPrecommitToken precommit_token = 4;
   }
+
+  // If `TransactionOptions.isolation_level` is set to
+  // `IsolationLevel.REPEATABLE_READ`, then the snapshot timestamp is the
+  // timestamp at which all reads in the transaction ran. This timestamp is
+  // never returned.
+  google.protobuf.Timestamp snapshot_timestamp = 5;
 }