add viz diagnostic metrics to prometheus-exporter

- Viz writes stats to /tmp/viz-stats.json (frames_sent, frames_dropped, queue_depth)
- Exporter reads viz-stats.json and exposes:
  - viz_frames_sent_total (counter)
  - viz_frames_dropped_total (counter)
  - viz_pending_queue (gauge)
  - viz_queue_full (gauge)

scripts/prometheus-exporter.py

@@ -4,7 +4,7 @@
 Exposes MIDI, audio, and service health metrics on :9090/metrics.
 No external dependencies — uses only stdlib.
 """
-import http.server, time, os, re, subprocess, socket, threading
+import http.server, time, os, re, subprocess, socket, threading, json
 
 PORT = int(os.environ.get("METRICS_PORT", "9090"))
 SSH_KEY = os.environ.get("SSH_KEY", "/home/david/.ssh/unoq_deploy_key")
@@ -24,11 +24,25 @@ metrics = {
     "synth_hphl_volume": 0,
     "synth_hphr_volume": 0,
     "synth_up": 1,
+    "viz_frames_sent": 0,
+    "viz_frames_dropped": 0,
+    "viz_pending_queue": 0,
+    "viz_queue_full": 0,
 }
 
 prev_hw_ptr = 0
 prev_hw_ptr_time = 0
 
+VIZ_STATS_FILE = "/tmp/viz-stats.json"
+
+
+def read_viz_stats():
+    try:
+        with open(VIZ_STATS_FILE) as f:
+            return json.load(f)
+    except (OSError, ValueError):
+        return {}
+
 
 def read_local_file(path):
     try:
@@ -106,6 +120,12 @@ def collect_metrics():
             metrics["synth_hphr_switch"] = get_mixer_value("HPHR Switch")
             metrics["synth_hphl_volume"] = get_mixer_value("HPHL Volume")
             metrics["synth_hphr_volume"] = get_mixer_value("HPHR Volume")
+
+            viz = read_viz_stats()
+            metrics["viz_frames_sent"] = viz.get("frames_sent", 0)
+            metrics["viz_frames_dropped"] = viz.get("frames_dropped", 0)
+            metrics["viz_pending_queue"] = viz.get("pending_queue", 0)
+            metrics["viz_queue_full"] = 1 if viz.get("queue_full", False) else 0
         except Exception:
             pass
 
@@ -158,6 +178,22 @@ def format_metrics():
     lines.append("# TYPE synth_up gauge")
     lines.append(f"synth_up {metrics['synth_up']}")
 
+    lines.append("# HELP viz_frames_sent_total Total frames sent to router")
+    lines.append("# TYPE viz_frames_sent_total counter")
+    lines.append(f"viz_frames_sent_total {metrics['viz_frames_sent']}")
+
+    lines.append("# HELP viz_frames_dropped_total Total frames dropped (queue full)")
+    lines.append("# TYPE viz_frames_dropped_total counter")
+    lines.append(f"viz_frames_dropped_total {metrics['viz_frames_dropped']}")
+
+    lines.append("# HELP viz_pending_queue Current pending frame queue depth")
+    lines.append("# TYPE viz_pending_queue gauge")
+    lines.append(f"viz_pending_queue {metrics['viz_pending_queue']}")
+
+    lines.append("# HELP viz_queue_full Router write buffer was full")
+    lines.append("# TYPE viz_queue_full gauge")
+    lines.append(f"viz_queue_full {metrics['viz_queue_full']}")
+
     return "\n".join(lines) + "\n"