Add parameter for specifying prometheus path (#1602)

Add a `--prometheus-path` parameter which allows users to specify at
what
path the metrics should be exposed.

This simplifies serving metrics on a specific path behind some
restrictive reverse proxies that expect the upstream server to serve
URLs with paths matching the requested path.

Co-authored-by: Pavel Punsky <eakraly@users.noreply.github.com>

examples/run_tests_prom.sh

@@ -64,3 +64,10 @@ turnserver_pid="$!"
 sleep 2
 assert_prom_response "http://127.0.0.1:8080/metrics"
 kill "$turnserver_pid"
+
+echo "Running turnserver with prometheus, using custom path"
+$BINDIR/turnserver --prometheus --prometheus-path="/coturn/metrics" > /dev/null &
+turnserver_pid="$!"
+sleep 2
+assert_prom_response "http://localhost:9641/coturn/metrics"
+kill "$turnserver_pid"

src/apps/relay/mainrelay.c

@@ -209,6 +209,7 @@ turn_params_t turn_params = {
     0,                                  /* prometheus disabled by default */
     DEFAULT_PROM_SERVER_PORT,           /* prometheus port */
     "",                                 /* prometheus address */
+    "/metrics",                         /* prometheus path */
     0, /* prometheus username labelling disabled by default when prometheus is enabled */
 
     ///////////// Users DB //////////////
@@ -1137,6 +1138,7 @@ static char Usage[] =
     "						also the path / on this port can be used as a health check\n"
     " --prometheus-port		<port>		Prometheus metrics port (Default: 9641).\n"
     " --prometheus-address		<address>		Prometheus listening address (Default: any).\n"
+    " --prometheus-path		<path>		Prometheus serve path (Default: /metrics).\n"
     " --prometheus-username-labels			When metrics are enabled, add labels with client usernames.\n"
 #endif
     " --use-auth-secret				TURN REST API flag.\n"
@@ -1436,6 +1438,7 @@ enum EXTRA_OPTS {
   PROMETHEUS_OPT,
   PROMETHEUS_PORT_OPT,
   PROMETHEUS_ADDRESS_OPT,
+  PROMETHEUS_PATH_OPT,
   PROMETHEUS_ENABLE_USERNAMES_OPT,
   AUTH_SECRET_OPT,
   NO_AUTH_PINGS_OPT,
@@ -1555,6 +1558,7 @@ static const struct myoption long_options[] = {
     {"prometheus", optional_argument, NULL, PROMETHEUS_OPT},
     {"prometheus-port", optional_argument, NULL, PROMETHEUS_PORT_OPT},
     {"prometheus-address", optional_argument, NULL, PROMETHEUS_ADDRESS_OPT},
+    {"prometheus-path", optional_argument, NULL, PROMETHEUS_PATH_OPT},
     {"prometheus-username-labels", optional_argument, NULL, PROMETHEUS_ENABLE_USERNAMES_OPT},
 #endif
     {"use-auth-secret", optional_argument, NULL, AUTH_SECRET_OPT},
@@ -2207,6 +2211,9 @@ static void set_option(int c, char *value) {
   case PROMETHEUS_ADDRESS_OPT:
     STRCPY(turn_params.prometheus_address, value);
     break;
+  case PROMETHEUS_PATH_OPT:
+    STRCPY(turn_params.prometheus_path, value);
+    break;
   case PROMETHEUS_ENABLE_USERNAMES_OPT:
     turn_params.prometheus_username_labels = 1;
     break;

src/apps/relay/mainrelay.h

@@ -311,6 +311,7 @@ typedef struct _turn_params_ {
   int prometheus;
   int prometheus_port;
   char prometheus_address[INET6_ADDRSTRLEN];
+  char prometheus_path[1025];
   int prometheus_username_labels;
 
   /////// Users DB ///////////

src/apps/relay/prom_server.c

@@ -52,7 +52,7 @@ MHD_RESULT promhttp_handler(void *cls, struct MHD_Connection *connection, const
   if (strcmp(method, "GET") != 0) {
     status = MHD_HTTP_METHOD_NOT_ALLOWED;
     body = "method not allowed";
-  } else if (strcmp(url, "/metrics") == 0) {
+  } else if (strcmp(url, turn_params.prometheus_path) == 0) {
     body = prom_collector_registry_bridge(PROM_COLLECTOR_REGISTRY_DEFAULT);
     mode = MHD_RESPMEM_MUST_FREE;
     status = MHD_HTTP_OK;