Feature Request: Support `TRANSACTION_TIMEOUT_MS` query hint

[mhamza15]

Feature Description

Similar to --queryserver-config-query-timeout and QUERY_TIMEOUT_MS, it would be nice to have a similar query hint for --queryserver-config-transaction-timeout like TRANSACTION_TIMEOUT_MS.

Use Case(s)

There has been an effort to reduce this timeout for our clusters, but doing it at a tablet/cluster granularity isn't ideal. Doing at the query level allows app teams to opt-in to the behavior on their own terms. For example, lowering the transaction timeout on web requests while reducing it more slowly for background jobs.

[mhamza15]

I think I'm mixing two things up: --queryserver-config-query-timeout is the vttablet level config, and it is actually the vtgate arg --query-timeout that is associated with QUERY_TIMEOUT_MS. I'm not too sure how transaction timeout could be applied in the same way, or if it even makes sense as a query hint 🤔 .

[arthurschreiber]

I think it would be better to have this as a session variable. Managing this through a query comment is awkward (e.g. if there's multiple queries inside a transaction that all set this via a comment, which one will be used, and how will this affect the already running transaction?)