Michael Usner

Deprecate lower-case 'prometheus_multiproc_dir'

1

The Prometheus Python client has deprecated the lower-case ```prometheus_multiproc_dir``` environment variable. This PR brings the check and warning for this condition into parity with https://github.com/prometheus/client_python/blob/9a6e21de144e81ac666828aa843efb398d184b27/prometheus_client/multiprocess.py#L28