res_prometheus: Do not generate broken metrics

In 8d6fdf9c3a invisible bridges were
skipped but that lead to producing metrics with no name and no help.

Keep track of the number of metrics configured and then only emit these.
Add a basic testcase that verifies that there is no '(NULL)' in the
output.

ASTERISK-30474

res/prometheus/bridges.c

@@ -83,7 +83,7 @@ static void bridges_scrape_cb(struct ast_str **response)
 	struct ast_bridge *bridge;
 	struct prometheus_metric *bridge_metrics;
 	char eid_str[32];
-	int i, j, num_bridges;
+	int i, j, num_bridges, num_outputs = 0;
 	struct prometheus_metric bridge_count = PROMETHEUS_METRIC_STATIC_INITIALIZATION(
 		PROMETHEUS_METRIC_GAUGE,
 		"asterisk_bridges_count",
@@ -138,7 +138,7 @@ static void bridges_scrape_cb(struct ast_str **response)
 		}
 
 		for (j = 0; j < ARRAY_LEN(bridge_metric_defs); j++) {
-			int index = i * ARRAY_LEN(bridge_metric_defs) + j;
+			int index = num_outputs++;
 
 			bridge_metrics[index].type = PROMETHEUS_METRIC_GAUGE;
 			ast_copy_string(bridge_metrics[index].name, bridge_metric_defs[j].name, sizeof(bridge_metrics[index].name));
@@ -159,7 +159,7 @@ static void bridges_scrape_cb(struct ast_str **response)
 	}
 	ao2_iterator_destroy(&it_bridges);
 
-	for (j = 0; j < ARRAY_LEN(bridge_metric_defs); j++) {
+	for (j = 0; j < num_outputs; j++) {
 		prometheus_metric_to_string(&bridge_metrics[j], response);
 	}
 

tests/test_res_prometheus.c

@@ -29,8 +29,11 @@
 
 #include "asterisk/test.h"
 #include "asterisk/module.h"
+#include "asterisk/bridge.h"
+#include "asterisk/bridge_basic.h"
 #include "asterisk/config.h"
 #include "asterisk/res_prometheus.h"
+#include "../res/prometheus/prometheus_internal.h"
 
 #define CATEGORY "/res/prometheus/"
 
@@ -699,6 +702,52 @@ AST_TEST_DEFINE(config_general_core_metrics)
 	return AST_TEST_PASS;
 }
 
+static void safe_bridge_destroy(struct ast_bridge *bridge)
+{
+	if (!bridge) {
+		return;
+	}
+	ast_bridge_destroy(bridge, 0);
+}
+
+AST_TEST_DEFINE(bridge_to_string)
+{
+	RAII_VAR(struct ast_bridge *, bridge1, NULL, safe_bridge_destroy);
+	RAII_VAR(struct ast_bridge *, bridge2, NULL, safe_bridge_destroy);
+	struct ast_str *response;
+
+	switch (cmd) {
+	case TEST_INIT:
+		info->name = __func__;
+		info->category = CATEGORY;
+		info->summary = "Test producing bridge metrics";
+		info->description =
+			"This test covers checking the metrics produced by the\n"
+			"bridge support of the basic Promtheus module.";
+		return AST_TEST_NOT_RUN;
+	case TEST_EXECUTE:
+		break;
+	}
+
+	bridge1 = ast_bridge_basic_new();
+	ast_test_validate(test, bridge1 != NULL);
+
+	bridge2 = ast_bridge_base_new(AST_BRIDGE_CAPABILITY_HOLDING,
+		AST_BRIDGE_FLAG_INVISIBLE,
+		"test_res_prometheus", "test_bridge_invisible", NULL);
+
+	response = prometheus_scrape_to_string();
+	if (!response) {
+		return AST_TEST_FAIL;
+	}
+
+	ast_test_status_update(test, " -> Retrieved: %s\n", ast_str_buffer(response));
+	ast_test_validate(test, strstr(ast_str_buffer(response), "(null)") == NULL);
+	ast_test_validate(test, strstr(ast_str_buffer(response), "asterisk_bridges_channels_count{") != NULL);
+	ast_free(response);
+	return AST_TEST_PASS;
+}
+
 static int process_config(int reload)
 {
 	struct ast_config *config;
@@ -791,6 +840,8 @@ static int unload_module(void)
 	AST_TEST_UNREGISTER(config_general_basic_auth);
 	AST_TEST_UNREGISTER(config_general_core_metrics);
 
+	AST_TEST_UNREGISTER(bridge_to_string);
+
 	return 0;
 }
 
@@ -813,6 +864,8 @@ static int load_module(void)
 	AST_TEST_REGISTER(config_general_basic_auth);
 	AST_TEST_REGISTER(config_general_core_metrics);
 
+	AST_TEST_REGISTER(bridge_to_string);
+
 	ast_test_register_init(CATEGORY, &test_init_cb);
 	ast_test_register_cleanup(CATEGORY, &test_cleanup_cb);