MB-72082: add more stats around vector search

[Thejas-bhat]

• introduce a new stats handler in the zap plugin. this would benefit in tracking stats all over the codebase using a single *Stats instance in the plugin object.
• currently this PR covers stats for the vector section on float indexes, but the handler can be passed around with new stats to other sections as well to get a better deep dive view of the zap layer.