Stream execute query logging

[arka-g]

Description

At Slack, we noticed that we're dropping logs for all queries of type STREAM_EXECUTE at the vttablet level. We run with the querylog-filter-tag option to determine what queries should be logged. The reason we were dropping all the logs for stream queries is because the Stream() codepath overwrites the OriginalSQL in logstats, which we rely on for checking if we should log this query here. This PR fixes that by not overwriting that field.

More details

The bug seems to be in the Stream vttablet method, we overwrite OriginalSQL to the version without the comments. In the callback that we pass to execRequest, we end up calling SplitMarginComments which strips comments before invoking Stream(). So, in the stream context, we don’t have the comments attached to the sql. This is fine because execRequest wraps the entire thing, and it has the full query with the comment.

I thought about two possible fixes:

1. append the leading and trailing comments when overwriting OriginalSQL inside Stream()
2. don’t overwrite OriginalSQL inside Stream(). this assignment seems unnecessary because execRequest initializes this properly and wraps the call. This PR implements this approach.

Testing

Added a unit test to ensure we preserve comments in the stream path.