Support Transaction Tags and Request Tags

[takabow]

This PR adds Cloud Spanner's transaction tags and request tags support to spanner-cli.

Transaction tags are tags set at the beginning of a transaction that makes the transaction identifiable. Request tags can be set for each request and identify individual requests.

According to this document, read-write transaction supports transaction tags and request tags. Read-only transaction only supports request tags.

So, I implemented this feature as follows.

Transaction Tags and Request Tags support in spanner-cli

In a read-write transaction, you can add arbitrary tags following BEGIN RW TAG <tags>.
spanner-cli adds the tags set in BEGIN RW TAG as transaction tags.
This tag will also be used as request tags within the transaction.

# Read-write transaction
# transaction_tag = tx1
+--------------------+
| BEGIN RW TAG tx1;  |
|                    |
| SELECT val         |
| FROM tab1      +-----request_tag = tx1
| WHERE id = 1;      |
|                    |
| UPDATE tab1        |
| SET val = 10   +-----request_tag = tx1
| WHERE id = 1;      |
|                    |
| COMMIT;            |
+--------------------+

In a read-only transaction, you can add arbitrary tags following BEGIN RO TAG <tags>.
Since read-only transaction in Cloud Spanner doesn't support transaction tags, spanner-cli adds the tags set in BEGIN RO TAG as request tags.

# Read-only transaction
# transaction_tag = N/A
+--------------------+
| BEGIN RO TAG tx2;  |
|                    |
| SELECT SUM(val)    |
| FROM tab1      +-----request_tag = tx2
| WHERE id = 1;      |
|                    |
| CLOSE;             |
+--------------------+

[yfuruyama]

Thank you @takabow for creating this change! PTAL the above comments.

[yfuruyama]

LGTM!