searcher: emit a histogram around the prepareZip access

Created by: coury-clark

I've been investigating some ideas to improve the sum search duration for a Code Insight, and wanted to be able to measure how long the zip access takes, specifically in the Searcher path. This just adds a metric that wraps the prepareZip in a histogram. I didn't add a dashboard yet, but if this ends up being useful I'll follow up with a new panel in another PR.

Test plan

I ran some unindexed searches locally and looked at the resulting metrics: