feat: add native Prometheus metrics exporter to stats server

Add a built-in Prometheus exposition format endpoint to the uWSGI stats
server, eliminating the need for external sidecar exporters like
uwsgi_exporter. Metric names are compatible with timonwong/uwsgi_exporter
for drop-in replacement.

GET /metrics returns Prometheus text format (42 metrics covering global,
socket, worker, app, core, and cache stats). All other paths return JSON
as before. The prometheus path is configurable via --stats-prometheus-path.

The HTTP stats layer is split into uwsgi_stats_read_request() and
uwsgi_stats_send_http_header() with an enum uwsgi_stats_format for
clean format selection.

Includes integration tests for both JSON and Prometheus endpoints.

Author: lavr

.gitignore

@@ -29,3 +29,6 @@ core/dot_h.c
 /uWSGI.egg-info/
 check/check_*
 !check/*.c
+
+# ralphex progress logs
+.ralphex/progress/

core/emperor.c

@@ -2528,7 +2528,12 @@ static void emperor_send_stats(int fd) {
 	}
 
 	if (uwsgi.stats_http) {
-		if (uwsgi_send_http_stats(client_fd)) {
+		enum uwsgi_stats_format fmt;
+		if (uwsgi_stats_read_request(client_fd, &fmt)) {
+			close(client_fd);
+			return;
+		}
+		if (uwsgi_stats_send_http_header(client_fd, UWSGI_STATS_FORMAT_JSON)) {
 			close(client_fd);
 			return;
 		}

core/stats.c

@@ -365,17 +365,33 @@ void uwsgi_send_stats(int fd, struct uwsgi_stats *(*func) (void)) {
 		return;
 	}
 
+	enum uwsgi_stats_format fmt = UWSGI_STATS_FORMAT_JSON;
 	if (uwsgi.stats_http) {
-		if (uwsgi_send_http_stats(client_fd)) {
+		if (uwsgi_stats_read_request(client_fd, &fmt)) {
 			close(client_fd);
 			return;
 		}
 	}
 
-	struct uwsgi_stats *us = func();
+	struct uwsgi_stats *us;
+	switch (fmt) {
+	case UWSGI_STATS_FORMAT_PROMETHEUS:
+		us = uwsgi_master_generate_stats_prometheus();
+		break;
+	default:
+		us = func();
+		break;
+	}
+
 	if (!us)
 		goto end;
 
+	if (uwsgi.stats_http) {
+		if (uwsgi_stats_send_http_header(client_fd, fmt)) {
+			goto end0;
+		}
+	}
+
 	size_t remains = us->pos;
 	off_t pos = 0;
 	while (remains > 0) {