CONTRIB: move prometheus-exporter to addons/promex

Let's start to better organize the addons by moving promex there (and
with an easier directory name). The makefile and maintainers files were
updated, as well as the CI's build matrix.
diff --git a/Makefile b/Makefile
index 2e15ed6..3a295c7 100644
--- a/Makefile
+++ b/Makefile
@@ -617,7 +617,7 @@
 endif
 
 ifneq ($(USE_PROMEX),)
-OPTIONS_OBJS    += contrib/prometheus-exporter/service-prometheus.o
+OPTIONS_OBJS    += addons/promex/service-prometheus.o
 endif
 
 ifneq ($(USE_DEVICEATLAS),)
@@ -999,10 +999,11 @@
 
 clean:
 	$(Q)rm -f *.[oas] src/*.[oas] haproxy test .build_opts .build_opts.new
-	$(Q)for dir in . src dev/* admin/* include/* doc; do rm -f $$dir/*~ $$dir/*.rej $$dir/core; done
+	$(Q)for dir in . src dev/* admin/* addons/* include/* doc; do rm -f $$dir/*~ $$dir/*.rej $$dir/core; done
 	$(Q)rm -f haproxy-$(VERSION).tar.gz haproxy-$(VERSION)$(SUBVERS)$(EXTRAVERSION).tar.gz
 	$(Q)rm -f haproxy-$(VERSION) haproxy-$(VERSION)$(SUBVERS)$(EXTRAVERSION) nohup.out gmon.out
 	$(Q)rm -f {admin,dev,contrib}/*/*.[oas] {admin,dev,contrib}/*/*/*.[oas] {admin,dev,contrib}/*/*/*/*.[oas]
+	$(Q)rm -f addons/promex/*.[oas]
 	$(Q)rm -f admin/iprange/iprange admin/iprange/ip6range admin/halog/halog
 	$(Q)rm -f dev/flags/flags dev/poll/poll dev/tcploop/tcploop
 	$(Q)rm -f dev/hpack/decode dev/hpack/gen-enc dev/hpack/gen-rht