Summary:
This diff adds vector_index_dump_stats flag.
When this flag is set to true, vector index search stats are added to the log on every vector index read query.
Jira: DB-15399

Test Plan: Jenkins

Reviewers: slingam

Reviewed By: slingam

Subscribers: ybase, yql

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D41931