Issue with prometheus_multiproc_dir deprecation

[tonial]

Hi !

I noticed an issue with prometheus_multiproc_dir deprecation in the latest 0.10.0 version.
In mark_process_dead() the environment variable transition code is not present, which means that if the uppercase version is not set, the function does not work.
https://github.com/prometheus/client_python/pull/624/files#diff-95ab27209a655ee3a508d436368ba4e1d155784c5885e18be09b61c2a5ec03efL155

Since we can call mark_process_dead() without going through the __init__() methods that set the new environment variable, it would required something like this : https://github.com/prometheus/client_python/pull/624/files#diff-f1ab5b765643c2beafab5bcd9874a839f7ebf080300e1ee0dbdb821755f8e904R56-R58

[csmarchbanks]

Hello, good catch! I see how that could be an issue if someone marks a process as dead without ever setting up a collector. Would you be interested in opening a PR? Otherwise I will fix it and release v0.10.1.

[romuald]

@csmarchbanks note that the README was not updated by the commit. I assume this is an oversight
(edit: also I realize I'm not in the correct issue .. sorry)

[csmarchbanks]

Since everything was supposed to work with either version I left the README alone for a bit until people had time to upgrade and not be confused if they were still running 0.9.0. There is a second PR (#628) to update the README that I plan to merge soon.

[tonial]

Oh ! I'm sorry @csmarchbanks, I should have made a PR right away when creating the issue.
I got sidetracked with other issues and forgot to come back and check your answer.

Thanks a lot for your very short reaction time to fix the issue :)