mcd_update_state metric should have a single time-series per node

Seen in build02, currently running 4.12.0-ec.3:

mcd_update_state{node="build0-gstfj-m-0.c.openshift-ci-build-farm.internal"}

returns:

Those are identical, except:

• The first has config populated with rendered-... and has a non-zero value.
• The second has config empty and has a zero value.

Looking at the backing code, my guess is that we're doing something like this:

• Things are happy; export with a populated config.
• Things get sad. Export with an empty config and a new error. But the happy time-series sticks around, and somehow has the value move to zero.
• Things get happy again; and we return to setting a value for the happy time-series. But the sad time-series sticks around, and somehow has the value move to zero.

Or something like that.  I expect we want to drop the zero-valued time-series, but I'm not clear enough on how the MCO pushes values into the export set to have code suggestions.