feat(metrics): pre-seed known channels at zero on startup

Without this, Prometheus only creates a series for cryptify_uploads_total
{channel="X"} when the first upload from channel X lands. PromQL
increase(...[1h]) over that series returns nothing because the earliest
sample is already non-zero — dashboards under-report low-volume channels
for an hour after each pod restart.

Pre-seeds website, staging-website, outlook, thunderbird, api, unknown
to 0 in Metrics::new(), so the full label set is visible from the first
scrape and increase() can compute deltas honestly.

The previous render-time "if empty, emit unknown=0" fallback is now
effectively dead (the map is never empty after new()), but left in place
defensively against any future code path that constructs Metrics via
Default rather than new().

Author: rubenhensen

src/metrics.rs

@@ -20,6 +20,20 @@ use rocket::http::HeaderMap;
 /// Channel label used when no other source information is present.
 pub const CHANNEL_UNKNOWN: &str = "unknown";
 
+/// Channels pre-seeded at value 0 on startup so dashboards see the full
+/// label set from the first scrape, rather than each channel popping into
+/// existence the first time a request from it lands. Without this, PromQL
+/// `increase()` over a window can read `0` for a channel whose first
+/// observed sample is already non-zero — see #102 follow-up discussion.
+pub const KNOWN_CHANNELS: &[&str] = &[
+    "website",
+    "staging-website",
+    "outlook",
+    "thunderbird",
+    "api",
+    CHANNEL_UNKNOWN,
+];
+
 /// Header clients can set to identify themselves (`outlook`, `thunderbird`,
 /// `api`, ...). Leading whitespace is trimmed and the value is lowercased
 /// and restricted to `[a-z0-9_-]` so it cannot inject Prometheus syntax.
@@ -36,7 +50,16 @@ pub struct Metrics {
 
 impl Metrics {
     pub fn new() -> Self {
-        Self::default()
+        let m = Self::default();
+        {
+            let mut uploads = m.uploads.lock().unwrap();
+            let mut bytes = m.upload_bytes.lock().unwrap();
+            for c in KNOWN_CHANNELS {
+                uploads.insert((*c).to_string(), 0);
+                bytes.insert((*c).to_string(), 0);
+            }
+        }
+        m
     }
 
     /// Record a successfully finalized upload.
@@ -325,11 +348,19 @@ mod tests {
     }
 
     #[test]
-    fn render_emits_zero_counters_when_empty() {
+    fn render_preseeds_known_channels_at_zero() {
         let m = Metrics::new();
         let text = m.render();
-        assert!(text.contains("cryptify_uploads_total{channel=\"unknown\"} 0"));
-        assert!(text.contains("cryptify_upload_bytes_total{channel=\"unknown\"} 0"));
+        for c in KNOWN_CHANNELS {
+            assert!(
+                text.contains(&format!("cryptify_uploads_total{{channel=\"{c}\"}} 0")),
+                "missing zero-seed for uploads channel={c} in:\n{text}"
+            );
+            assert!(
+                text.contains(&format!("cryptify_upload_bytes_total{{channel=\"{c}\"}} 0")),
+                "missing zero-seed for upload_bytes channel={c} in:\n{text}"
+            );
+        }
         assert!(text.contains("cryptify_storage_bytes 0"));
         assert!(text.contains("cryptify_active_files 0"));
         assert!(text.contains("cryptify_expired_files_total 0"));