Log as JSON when IPX_LOG_FORMAT=json (#91)

The log was text, so the Grafana dashboard picked lines apart with
regular expressions, and a change of wording would have blanked its
panels. With IPX_LOG_FORMAT=json each line is one JSON object: the
access log carries method, path, route, status and ms as fields (the
route passed from the routing layer in the response's extensions), and
each wire event its ev, feed, new, downloaded, failed, bytes, msg and
the rest (log_wire), beside the old message. The two startup lines that
were println! are logged, so no line breaks the JSON. Text stays the
default, for a terminal. The dashboard reads the fields with Loki's json
parser, and groups requests by route rather than path.

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

src/main.rs

@@ -183,19 +183,32 @@ async fn main() -> Result<()> {
         // Two filters, deliberately different. stderr follows IPX_LOG; the in-app buffer
         // keeps debug as well, so the log view can show protocol traffic and routine
         // skips that would be noise on a terminal. IPX_UI_LOG overrides it.
-        let stderr_filter = tracing_subscriber::EnvFilter::try_from_env("IPX_LOG")
-            .unwrap_or_else(|_| "ipx=info".into());
+        let stderr_filter = || {
+            tracing_subscriber::EnvFilter::try_from_env("IPX_LOG").unwrap_or_else(|_| "ipx=info".into())
+        };
+        // One JSON object a line for Loki (#91), with each event's fields as its own; text
+        // otherwise, for someone reading a terminal.
+        let json = std::env::var("IPX_LOG_FORMAT").is_ok_and(|f| f.eq_ignore_ascii_case("json"));
         let ui_filter = tracing_subscriber::EnvFilter::try_from_env("IPX_UI_LOG")
             .unwrap_or_else(|_| "ipx=debug".into());
         tracing_subscriber::registry()
-            .with(
+            .with((!json).then(|| {
                 tracing_subscriber::fmt::layer()
                     .with_writer(std::io::stderr)
                     // Colour for a terminal only: in docker logs and Loki the escapes are noise
                     // every query has to strip (#88).
                     .with_ansi(std::io::IsTerminal::is_terminal(&std::io::stderr()))
-                    .with_filter(stderr_filter),
-            )
+                    .with_filter(stderr_filter())
+            }))
+            .with(json.then(|| {
+                tracing_subscriber::fmt::layer()
+                    .json()
+                    .flatten_event(true)
+                    .with_current_span(true)
+                    .with_span_list(false)
+                    .with_writer(std::io::stderr)
+                    .with_filter(stderr_filter())
+            }))
             .with(logbuf::RingLayer.with_filter(ui_filter))
             .with(otel.as_ref().map(|p| {
                 use opentelemetry::trace::TracerProvider;
@@ -575,12 +588,13 @@ async fn start_web(
         ctx.set_cfg(fresh.clone());
         // The token signs in as the admin, and whatever reads this process's output (docker logs,
         // for one) is wider than who reads config.toml. So say where it is, never what it is.
-        println!(
+        // Logged, not printed, so a JSON log stays one object a line (#91).
+        tracing::info!(
             "web ui token generated and saved to {} as [web] token. Open http://{bind}/?token=<that token>",
             config_path.display()
         );
     } else {
-        println!(
+        tracing::info!(
             "web ui at http://{bind}/ (the sign-in token is [web] token in {})",
             config_path.display()
         );