MINOR: contrib/prometheus-exporter: Add a param to ignore servers in maintenance

By passing the parameter "no-maint" in the query-string, it is now possible to
ignore servers in maintenance. It means that the metrics for servers in this
state will not be exported.

(cherry picked from commit eba2294e5b82931fd2e74f829e8092366728a6af)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
2 files changed