Summary:
Original commit: 3aa0773f89e47d5fdf341860241913988f7b54fd / D45447
transaction.cc is meant to log the transaction's trace whenever the time taken by the transaction is `> FLAGS_txn_slow_op_threshold_ms`

However, after the refactor in a0e6d550eb268818f9189d79d3066a61cba10be4
The log line does not special case the fact that the flag being 0 should disable printing the log line.

This causes log spew as `FLAGS_txn_slow_op_threshold_ms` defaults to 0.

We should only consider printing the trace if this flag is non-zero.

Also includes a4dc4d30db7c01a65d245f2e5d84308754a650b3 / D45498
Jira: DB-17617, DB-17653

Test Plan: yb_build.sh fastdebug --cxx-test pg_mini-test --gtest_filter PgMiniTestTracing/PgMiniTestTracing.Tracing/*

Reviewers: rthallam, bkolagani

Reviewed By: bkolagani

Subscribers: yql, ybase

Differential Revision: https://phorge.dev.yugabyte.com/D45571