When metrics are pre-registered with Help and without labels,
when issuing metrics with labels, the help was filled with
the key_name.

But prometheus client enforces help to be consistent accross
several elements, so, it can break at Runtime if a metric
has been declared without its labels and later issued this metric
with labels.

In order to fix this, we store the help declared at startup for
each key, so, when declaring a metric with some labels, we try
to re-used pre-declared help for the metric.

It should fix the issue that appeared with Consul 1.9.x and
fix hashicorp/consul#9471

…_registered_help

Use the same Help for metrics with and without labels