BUG/MINOR: contrib/prometheus-exporter: decode parameter and value only

BUG/MINOR: contrib/prometheus-exporter: decode parameter and value only

we were decoding all substring and then parsing; this could lead to
consider & and = in decoding result as delimiters where it should not.
this patch reverses the order by first parsing and then decoding each key
and value separately.

we also stop parsing after number sign (#).

This patch should be backported to 2.1 and 2.0

Signed-off-by: William Dauchy <w.dauchy@criteo.com>
(cherry picked from commit c65f656d75091db3087a752dbc956159392fc8f2)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>