Summary:
New changes-
- Since query diagnostics is not enabled, used pg cron to test the bg worker changes

Original commit: 8d20fda8b0e073b2aca93ce80298f9e570356d30 / D41832
This diff sets the query id for background workers much
earlier, so that all samples of background workers would
have the query id of 7.
Jira: DB-15276

Test Plan: ./yb_build.sh --cxx-test pg_ash-test --gtest_filter PgBgWorkersTest.TestBgWorkersQueryId

Reviewers: ishan.chhangani, hbhanawat

Reviewed By: ishan.chhangani

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D41892