MAJOR: contrib/prometheus-exporter: move ftd/bkd/srv states to labels

MAJOR: contrib/prometheus-exporter: move ftd/bkd/srv states to labels

this patch is a breaking change between v2.3 and v2.4: we move from
using gauge value for frontend/backend/servers states to labels values.

the main motivation being I realised it is very difficult to make use of
it without hard coded quirks on prometheus client side; especially
because the main use is often to group by state, which is harder when
the state is the value of the metric.

in order to achieve that we iterate on the status metric to generate
labels, and so as many metrics.

this is the first step to resolve github issue #1029
A second step should address health check states.

Signed-off-by: William Dauchy <wdauchy@gmail.com>