Add an `iframes` custom metric

[adamsilverstein]

Based on the images metric, this one enumerates iframes present on the page including their:

• url
• width & height
• naturalWidth and naturalHeight (for example when when w/h altered by CSS)
• inViewport

[tunetheweb]

Most of this already exists in the almanac custom metric: https://github.com/HTTPArchive/custom-metrics/blob/main/metric-summary.md#iframes

We should enhance that. Though maybe there’s a good argument to move it to its own file while doing that. WDYT @rviscomi ?

[rviscomi]

I'd like to move more towards grouping custom metrics by chapter, ie markup.js for things like this.

[tunetheweb]

Reminder @adamsilverstein . Wanna update the almanac one short term? Or take on the moving of this to markup.js at the same time as doing your updates?