tracing: update CloudSpanner.Transaction.execute_streaming_sql to execute_sql; start Transaction.commit span up top to ensure

[odeke-em]

An issue for myself to fix requests from our last review session

• update "CloudSpanner.Transaction.execute_streaming_sql" span name to "execute_sql"
• Starting the CloudSpanner.Transaction.commit span before the inlined .begin()
• Ensure that using a tracer_provider with ALWAYS_OFF set doesn't crash due to NonRecording span not having _span

-            if (not span._status) or span._status.status_code == StatusCode.UNSET:
+            if getattr(span, '_status', None) is None or span._status.status_code == StatusCode.UNSET:

Thanks @harshachinta @surbhigarg92 @sakthivelmanii