Trace each step of the feed list (#164)

GET /api/feeds took 250-300 ms in production with nothing under it in its trace but feed_summary
spans of a millisecond, so there was no telling where the time went. feed_rows now wraps the
catalogue, the person's subscriptions, the subscriber counts, the pins and feed_list in a step
span each, and feed_list each of its five queries.

db::step makes a span only inside one already: the healthcheck's status every 30 s and the page's
live updates run the same code outside any trace, where each span would be a trace of its own, as
feed_summary's already are.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/web.rs

@@ -807,20 +807,24 @@ async fn feeds(
 /// The feed list as this person sees it, or with `only` the one row for that feed: what a live
 /// update sends after the feed changes, so the page redraws a row instead of reloading the list.
 async fn feed_rows(state: &WebState, user: &crate::db::User, only: Option<&str>) -> anyhow::Result<Vec<FeedRow>> {
+    use crate::db::step;
+    use tracing::Instrument;
     let cfg = state.ctx.cfg();
     // Config entries plus the feeds derived from OPML subscriptions -- the catalogue.
     // What comes back is only the part of it this person subscribes to.
-    let subs = crate::subscriptions(&state.ctx).await?;
+    let subs = crate::subscriptions(&state.ctx).instrument(step("catalogue")).await?;
     let mine: std::collections::HashMap<String, crate::db::Sub> = state
         .ctx
         .db
-        .subscriptions_for(user.id).await?
+        .subscriptions_for(user.id)
+        .instrument(step("subscriptions"))
+        .await?
         .into_iter()
         .map(|s| (s.feed_id.clone(), s))
         .collect();
-    let counts = state.ctx.db.subscriber_counts().await?;
-    let pinned = state.ctx.db.pinned_feeds(user.id).await?;
-    let mut listed = state.ctx.db.feed_list(user.id, only).await?;
+    let counts = state.ctx.db.subscriber_counts().instrument(step("subscriber_counts")).await?;
+    let pinned = state.ctx.db.pinned_feeds(user.id).instrument(step("pinned")).await?;
+    let mut listed = state.ctx.db.feed_list(user.id, only).instrument(step("feed_list")).await?;
     let mut out = Vec::with_capacity(mine.len());
     for sub in subs.iter().filter(|s| only.is_none_or(|o| o == s.id)) {
         let (id, feed) = (&sub.id, &sub.cfg);