Dupe v1 metrics API into equivalent v2 page

Each API endpoint page ought to be complete. We can say it's the same,
but bust still say what it actually is.
Fix headers and markdown in query endpoint pages

For great justice and/or compliance with our finicky table-of-contents plugin.
:parameter --> <PARAMETER> in abstract API endpoint names

According to Nick Lewis, this notation arose out of the way you define
parameters in a route in Sinatra, but I am of the opinion that that's a:
too implementation-specific, and b: an alien implementation in the first
place.

* Use standard placeholders.
* Make sure they're wrapped in code span backticks so they don't explode the page.