Remove get_counter_names from the pollster plugins

Now that there is only one meter-type coming out of
each plugin, we can use the plugin name instead of
asking the plugin which counters it produces.

blueprint one-meter-per-plugin

Change-Id: I63328b1bb2a2440771d9f1f007ff69e717d3090f
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>
Merge "Remove get_counter_names from the pollster plugins"