prometheus: Update default tsdb directory handling

Use /srv/prometheus instead of /data, because user `prometheus` doens't
have permissions to create `/data/` in `/`.

Instead this commit puts prometheus data into `/srv/prometheus` by
default, which is a cleaner path, and it'll create tsdb path & then assign
required permissions on each prometheus service start.

This way, also, the cases when users re-configure tsdb to point to
external USB - it'll also be created and assigned required permissions for
prometheus.

Signed-off-by: Alex Simkin <sashasimkin@gmail.com>

utils/prometheus/Makefile

@@ -47,6 +47,9 @@ Prometheus, a Cloud Native Computing Foundation project, is a systems and
 service monitoring system. It collects metrics from configured targets at given
 intervals, evaluates rule expressions, displays the results, and can trigger
 alerts if some condition is observed to be true.
+
+Default tsdb path is /srv/prometheus, you might want to edit /etc/config/prometheus
+in order to place it on USB storage or external SD card.
 endef
 
 define Package/prometheus/install

utils/prometheus/files/etc/init.d/prometheus

@@ -12,8 +12,14 @@ start_service() {
 	local web_listen_address
 	config_load "prometheus"
 	config_get config_file prometheus config_file "$CONFFILE"
-	config_get storage_tsdb_path prometheus storage_tsdb_path "/data"
+	config_get storage_tsdb_path prometheus storage_tsdb_path "/srv/prometheus"
 	config_get web_listen_address prometheus web_listen_address "127.0.0.1:9090"
+	
+	# Create tsdb dir & permissions if needed
+	if [ ! -d "$storage_tsdb_path" ]; then
+		mkdir "$storage_tsdb_path"
+		chown prometheus:prometheus "$storage_tsdb_path"
+	fi;
 
 	procd_open_instance
 	procd_set_param command "$PROG"

utils/prometheus/files/etc/uci-defaults/prometheus-defaults

@@ -6,7 +6,7 @@ uci -q get prometheus.prometheus || {
 	uci -q batch <<EOF
 	set prometheus.prometheus=prometheus
 	set prometheus.prometheus.config_file='/etc/prometheus.yml'
-	set prometheus.prometheus.storage_tsdb_path='/data'
+	set prometheus.prometheus.storage_tsdb_path='/srv/prometheus'
 	set prometheus.prometheus.web_listen_address='127.0.0.1:9090'
 	commit prometheus
 EOF