feat(metrics): pre-seed known channels at zero on startup (#165)

* 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().

* refactor(metrics): route Default through new() so they can't diverge

Address dobby's non-blocking observation on #165: with #[derive(Default)]
still in place, Metrics::default() produced an empty-channels object
while Metrics::new() pre-seeded — a footgun for a future contributor
who reaches for ::default() in a generic context or test helper.

Drops the derive, refactors new() to build the maps directly (no
internal lock dance), and adds a manual `impl Default for Metrics`
that just calls Self::new(). The two construction paths now produce
identical objects.

All 13 metrics tests still pass.

Author: rubenhensen

src/metrics.rs

@@ -20,12 +20,25 @@ 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.
 pub const SOURCE_HEADER: &str = "X-Cryptify-Source";
 
-#[derive(Default)]
 pub struct Metrics {
     uploads: Mutex<BTreeMap<String, u64>>,
     upload_bytes: Mutex<BTreeMap<String, u64>>,
@@ -34,9 +47,32 @@ pub struct Metrics {
     expired_files: AtomicU64,
 }
 
+// `Default` is implemented manually (not derived) so it goes through
+// `Metrics::new()` and pre-seeds `KNOWN_CHANNELS`. A derived `Default`
+// would silently produce an empty-channel object, which diverges from
+// `new()` and re-introduces the missing-baseline problem this module
+// exists to solve.
+impl Default for Metrics {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl Metrics {
     pub fn new() -> Self {
-        Self::default()
+        let mut uploads = BTreeMap::new();
+        let mut bytes = BTreeMap::new();
+        for c in KNOWN_CHANNELS {
+            uploads.insert((*c).to_string(), 0u64);
+            bytes.insert((*c).to_string(), 0u64);
+        }
+        Self {
+            uploads: Mutex::new(uploads),
+            upload_bytes: Mutex::new(bytes),
+            storage_bytes: AtomicI64::new(0),
+            active_files: AtomicI64::new(0),
+            expired_files: AtomicU64::new(0),
+        }
     }
 
     /// Record a successfully finalized upload.
@@ -325,11 +361,19 @@ 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"));