Replace transmission-exporter with homegrown Python exporter (#283)

## Summary
- Replace unmaintained `metalmatze/transmission-exporter:master` sidecar with a homegrown Python exporter
- Uses `prometheus_client` + `transmission-rpc` with collect-on-scrape pattern (fresh metrics per scrape, no stale labels)
- Same metric names so existing Grafana Transmission dashboard works unchanged
- Container built with `uv` for dependency management, follows `grafana-sidecar` Dockerfile pattern

## Changes
- **New:** `containers/transmission-exporter/exporter.py` — single-file exporter (~130 lines)
- **New:** `containers/transmission-exporter/Dockerfile` — multi-stage Alpine build with uv
- **Modified:** `argocd/manifests/torrent/deployment.yaml` — swap sidecar image reference
- **Modified:** `argocd/manifests/torrent/kustomization.yaml` — add image tag entry
- **Modified:** `service-versions.yaml` — add transmission-exporter entry

## Deployment and Testing
- [ ] Build container: `mise run container-build-and-release transmission-exporter`
- [ ] Update kustomization.yaml newTag with build SHA
- [ ] Branch deploy: `argocd app set torrent --revision feature/transmission-exporter-python && argocd app sync torrent`
- [ ] Verify metrics: `kubectl -n torrent --context=minikube-indri port-forward svc/transmission 19091:19091` then `curl localhost:19091/metrics | grep transmission_`
- [ ] Verify Grafana Transmission dashboard panels populate
- [ ] After merge: `argocd app set torrent --revision main && argocd app sync torrent`

Reviewed-on: #283

containers/transmission-exporter/Dockerfile

@@ -0,0 +1,21 @@
+# Transmission Prometheus exporter - collect-on-scrape, no polling loop
+# uv run --script handles dependency resolution at runtime
+
+ARG CONTAINER_APP_VERSION=1.0.0
+
+FROM python:3.13-alpine3.23
+
+LABEL org.opencontainers.image.title="Transmission Exporter"
+LABEL org.opencontainers.image.description="Prometheus exporter for Transmission BitTorrent client"
+
+COPY --from=ghcr.io/astral-sh/uv:latest /uv /usr/local/bin/uv
+
+ENV PYTHONUNBUFFERED=1
+ENV UV_CACHE_DIR=/tmp/uv-cache
+
+WORKDIR /app
+COPY exporter.py .
+
+EXPOSE 19091
+USER 65534:65534
+CMD ["uv", "run", "--script", "/app/exporter.py"]

containers/transmission-exporter/exporter.py

@@ -0,0 +1,150 @@
+#!/usr/bin/env -S uv run --script
+# /// script
+# requires-python = ">=3.13"
+# dependencies = [
+#     "prometheus-client",
+#     "transmission-rpc",
+# ]
+# ///
+"""Minimal Prometheus exporter for Transmission, using collect-on-scrape."""
+
+import os
+import sys
+import urllib.parse
+from wsgiref.simple_server import make_server
+
+from prometheus_client import make_wsgi_app
+from prometheus_client.core import REGISTRY, GaugeMetricFamily
+from transmission_rpc import Client
+
+
+def parse_addr(addr: str) -> dict:
+    """Parse TRANSMISSION_ADDR into kwargs for transmission_rpc.Client."""
+    parsed = urllib.parse.urlparse(addr)
+    kwargs: dict = {}
+    if parsed.hostname:
+        kwargs["host"] = parsed.hostname
+    if parsed.port:
+        kwargs["port"] = parsed.port
+    if parsed.scheme == "https":
+        kwargs["protocol"] = "https"
+    if parsed.path and parsed.path != "/":
+        kwargs["path"] = parsed.path.strip("/")
+    if parsed.username:
+        kwargs["username"] = parsed.username
+    if parsed.password:
+        kwargs["password"] = parsed.password
+    return kwargs
+
+
+class TransmissionCollector:
+    def __init__(self, client_kwargs: dict):
+        self._client_kwargs = client_kwargs
+
+    def collect(self):
+        try:
+            client = Client(**self._client_kwargs)
+            session = client.session_stats()
+            torrents = client.get_torrents()
+        except Exception as e:
+            print(f"Error collecting metrics: {e}", file=sys.stderr)
+            return
+
+        yield _gauge(
+            "transmission_session_stats_download_speed_bytes",
+            "Current download speed in bytes/s",
+            session.download_speed,
+        )
+        yield _gauge(
+            "transmission_session_stats_upload_speed_bytes",
+            "Current upload speed in bytes/s",
+            session.upload_speed,
+        )
+        yield _gauge(
+            "transmission_session_stats_torrents_active",
+            "Number of active torrents",
+            session.active_torrent_count,
+        )
+        yield _gauge(
+            "transmission_session_stats_torrents_total",
+            "Total number of torrents",
+            session.torrent_count,
+        )
+
+        downloaded = GaugeMetricFamily(
+            "transmission_session_stats_downloaded_bytes",
+            "Total bytes downloaded",
+            labels=["type"],
+        )
+        downloaded.add_metric(["cumulative"], session.cumulative_stats.downloaded_bytes)
+        yield downloaded
+
+        uploaded = GaugeMetricFamily(
+            "transmission_session_stats_uploaded_bytes",
+            "Total bytes uploaded",
+            labels=["type"],
+        )
+        uploaded.add_metric(["cumulative"], session.cumulative_stats.uploaded_bytes)
+        yield uploaded
+
+        t_download = GaugeMetricFamily(
+            "transmission_torrent_download_bytes",
+            "Torrent total downloaded bytes",
+            labels=["name"],
+        )
+        t_upload = GaugeMetricFamily(
+            "transmission_torrent_upload_bytes",
+            "Torrent total uploaded bytes",
+            labels=["name"],
+        )
+        t_ratio = GaugeMetricFamily(
+            "transmission_torrent_ratio",
+            "Torrent upload ratio",
+            labels=["name"],
+        )
+        t_uploaded_ever = GaugeMetricFamily(
+            "transmission_torrent_uploaded_ever",
+            "Torrent total uploaded ever in bytes",
+            labels=["name"],
+        )
+        t_done = GaugeMetricFamily(
+            "transmission_torrent_done",
+            "Torrent percent done (0.0-1.0)",
+            labels=["name"],
+        )
+
+        for t in torrents:
+            name = t.name or "unknown"
+            t_download.add_metric([name], t.total_size * t.percent_done)
+            t_upload.add_metric([name], t.uploaded_ever)
+            t_ratio.add_metric([name], t.ratio)
+            t_uploaded_ever.add_metric([name], t.uploaded_ever)
+            t_done.add_metric([name], t.percent_done)
+
+        yield t_download
+        yield t_upload
+        yield t_ratio
+        yield t_uploaded_ever
+        yield t_done
+
+
+def _gauge(name: str, doc: str, value: float) -> GaugeMetricFamily:
+    g = GaugeMetricFamily(name, doc)
+    g.add_metric([], value)
+    return g
+
+
+def main():
+    addr = os.environ.get("TRANSMISSION_ADDR", "http://localhost:9091")
+    port = int(os.environ.get("EXPORTER_PORT", "19091"))
+
+    client_kwargs = parse_addr(addr)
+    REGISTRY.register(TransmissionCollector(client_kwargs))
+
+    print(f"Listening on :{port}, scraping {addr}")
+    httpd = make_server("", port, make_wsgi_app())
+    httpd.serve_forever()
+
+
+if __name__ == "__main__":
+    main()