Remove outdated doc from collector middleware
Remove outdated doc from collector middleware

Also describe `:metrics_prefix` option.

Signed-off-by: Shouichi Kamiya <91f722587e4da4a3739ceec316dff53789465d8e@gmail.com>
Co-Authored-By: Daniel Magliola <349b614faf8a67c5438eb3fc104c4bf440ac9ea6@crystalgears.com>

prometheus/client_ruby