collector: clean up DRBD collector, less global state
collector: clean up DRBD collector, less global state

Signed-off-by: Matt Layher <76528ff2e917253aa2211f68e4ca60b5b846dcf5@gmail.com>

prometheus/node_exporter,siavashs/node_exporter,siavashs/node_exporter,siavashs/node_exporter,prometheus/node_exporter,siavashs/node_exporter,prometheus/node_exporter