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/.github/matrix.py b/.github/matrix.py
index 28b697f..e31d8ed 100644
--- a/.github/matrix.py
+++ b/.github/matrix.py
@@ -81,7 +81,7 @@
                 "WURFL_LIB=contrib/wurfl",
                 "USE_DEVICEATLAS=1",
                 "DEVICEATLAS_SRC=contrib/deviceatlas",
-                "EXTRA_OBJS=contrib/prometheus-exporter/service-prometheus.o",
+                "USE_PROMEX=1",
                 "USE_51DEGREES=1",
                 "51DEGREES_SRC=contrib/51d/src/pattern",
             ],
@@ -147,7 +147,7 @@
             "WURFL_LIB=contrib/wurfl",
             "USE_DEVICEATLAS=1",
             "DEVICEATLAS_SRC=contrib/deviceatlas",
-            "EXTRA_OBJS=contrib/prometheus-exporter/service-prometheus.o",
+            "USE_PROMEX=1",
             "USE_51DEGREES=1",
             "51DEGREES_SRC=contrib/51d/src/pattern",
         ],
diff --git a/MAINTAINERS b/MAINTAINERS
index 2497a36..7f4881d 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -119,7 +119,7 @@
 Prometheus Exporter
 Maintainer: Christopher Faulet <cfaulet@haproxy.com>
 Maintainer: William Dauchy <wdauchy@gmail.com>
-Files: contrib/prometheus-exporter
+Files: addons/promex
 Note: William is the referent regarding Prometheus. He should be consulted for
       all additions and modifications of features. Christopher is the referent
       for the code itself. He should be consulted for questions regarding the
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
diff --git a/contrib/prometheus-exporter/README b/addons/promex/README
similarity index 100%
rename from contrib/prometheus-exporter/README
rename to addons/promex/README
diff --git a/contrib/prometheus-exporter/service-prometheus.c b/addons/promex/service-prometheus.c
similarity index 100%
rename from contrib/prometheus-exporter/service-prometheus.c
rename to addons/promex/service-prometheus.c