feat: support transaction and request tags in dbapi

Adds support for setting transaction tags and request tags in dbapi.
This makes these options available to frameworks that depend on
dbapi, like SQLAlchemy and Django.

Towards #525

google/cloud/sqlalchemy_spanner/sqlalchemy_spanner.py

@@ -180,6 +180,10 @@ def pre_exec(self):
         if priority is not None:
             self._dbapi_connection.connection.request_priority = priority
 
+        request_tag = self.execution_options.get("request_tag")
+        if request_tag:
+            self.cursor.request_tag = request_tag
+
     def fire_sequence(self, seq, type_):
         """Builds a statement for fetching next value of the sequence."""
         return self._execute_scalar(

test/mockserver_tests/test_basics.py

@@ -71,7 +71,9 @@ def test_sqlalchemy_select1(self):
         with engine.connect().execution_options(
             isolation_level="AUTOCOMMIT"
         ) as connection:
-            results = connection.execute(select(1)).fetchall()
+            results = connection.execute(
+                select(1).execution_options(request_tag="my-tag")
+            ).fetchall()
             self.verify_select1(results)
 
     def test_sqlalchemy_select_now(self):