Prometheus: prevented caching of metric values.

Notably, the "angie_http/stream_upstreams_peers_state" metrics in the
"prometheus_all.conf" template could output the same value cached from
the first evaluation of $p8st_all_ups_state for two reasons:

 1. By default, the "map" evaluation was always cached.

 2. Even non-cacheable variable values weren't flushed properly.

Meanwhile, other metrics that use $p8s_value directly worked fine only
because the "valid" flag was not set.

--HG--
branch : 1.3

conf/prometheus_all.conf

@@ -346,6 +346,7 @@ angie_connections_idle $p8s_value
 }
 
 map $p8s_value $p8st_all_ups_state {
+    volatile;
     "up"           1;
     "down"         2;
     "unavailable"  3;

docs/xml/angie/changes.xml

@@ -7,6 +7,22 @@
 
 <changes ver="1.3.2" date="">
 
+<change type="bugfix">
+<para lang="ru">
+были возможны некорректные значения метрик в формате Prometheus, в значениях
+которых использовались отличные от $p8s_value переменные; на практике
+проблема могла наблюдаться с "angie_http_upstreams_peers_state"
+и "angie_stream_upstreams_peers_state" из стандартного шаблона
+"prometheus_all.conf".
+</para>
+<para lang="en">
+possible incorrect values of metrics in Prometheus output that used variables
+other than $p8s_value for their values; in practice the issue could occur with
+"angie_http_upstreams_peers_state" and "angie_stream_upstreams_peers_state"
+from the standard "prometheus_all.conf" template.
+</para>
+</change>
+
 <change type="bugfix">
 <para lang="ru">
 некоторые попытки соединения с проксируемыми серверами могли не учитываться

src/http/modules/ngx_http_prometheus_module.c

@@ -363,7 +363,7 @@ ngx_http_prometheus_render_metric(ngx_uint_t m,
 
     metric = ctx->metrics->elts;
 
-    vv = ngx_http_get_indexed_variable(ctx->request, metric[m].value);
+    vv = ngx_http_get_flushed_variable(ctx->request, metric[m].value);
 
     if (vv == NULL || vv->not_found || vv->len == 0) {
         return NGX_OK;
@@ -558,8 +558,6 @@ ngx_http_prometheus_variable(ngx_http_request_t *r,
         return NGX_OK;
     }
 
-    v->not_found = 0;
-
     switch (item->type) {
 
     case NGX_DATA_INTEGER_TYPE:
@@ -597,6 +595,9 @@ ngx_http_prometheus_variable(ngx_http_request_t *r,
         return NGX_ERROR;
     }
 
+    v->valid = 1;
+    v->not_found = 0;
+
     return NGX_OK;
 }