Apparently, a decision was made at some point to only use the v prefix
in tags and similar contexts where other things can appear. There was a
vote to stick to that decision. For more information, read
https://github.com/prometheus/prometheus/pull/2743.

This reverts commit 5405a472.

Sadly, this is urgently required.

This got dropped in the cherrypicking.

It now follows the policy of only documenting user-relevant changes.

Note that this will be merged into the release-0.19 branch and only
includes bug fixes, not other recent commits.

This is low-risk and contains critical bug fixes and should therefore
be released ASAP.