pgx transaction example improvements

[abh]

Version

1.28.0

What happened?

The example for how to do transactions with pgx added in #3557 will always run Rollback. The rollback will return an error which appears silent, but isn't if pgx is wrapped in opentelemetry tracing.

Instead of always doing a Rollback I believe you can make a context.WithCancel context and then defer cancel() will rollback the transaction if it's not already committed.

Relevant log output

Database schema

SQL queries

Configuration

Playground URL

No response

What operating system are you using?

No response

What database engines are you using?

No response

What type of code are you generating?

No response