fix(telemetry): start telemetry + metrics before [rpc_startup]

Metric-emitting code that first runs during ApplicationImp::setup() (the
[rpc_startup] loop) executed while the OTel meter was still empty, because
telemetry_ and metricsRegistry_ were started later in start(). A call-site
metric macro caches its instrument on first use via std::call_once, so that
first pre-meter use latched a null instrument for the process lifetime and the
metric silently never recorded (observed: rpc_in_flight_requests, emitted from
PerfLogImp::rpcStart, never appeared).

Extract telemetry_->start() and the metricsRegistry_->start() block into a new
private ApplicationImp::startTelemetry() and call it from setup() just before
the [rpc_startup] loop, so the meter is live before any metric-emitting code
runs. The call sits after overlay_ (and the other subsystems the observable-
gauge callbacks read) is constructed, since starting the metrics reader thread
earlier would let its callbacks call getOverlay() before overlay_ exists.
perfLog_->start() stays in start() — the macro call sites do not depend on it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/xrpld/app/main/Application.cpp

@@ -1163,6 +1163,29 @@ private:
     void
     startGenesisLedger();
 
+    /**
+     * Start the tracing and metrics pipelines.
+     *
+     * Called once from setup(), just before the [rpc_startup] loop. Starting
+     * here (rather than in start()) guarantees the OTel MeterProvider is live
+     * before any metric-emitting code runs — including startup RPCs, whose
+     * PerfLog instrumentation records a call-site metric. A call-site metric
+     * macro caches its instrument on first use via std::call_once; if that
+     * first use happens while the meter is still empty, the instrument latches
+     * null for the process lifetime and the metric silently never records.
+     *
+     * The call site sits after overlay_ and the other subsystems are
+     * constructed, because the metrics reader thread starts here and its
+     * observable-gauge callbacks read that state (e.g. getOverlay()); starting
+     * earlier would let the reader observe a half-built application.
+     *
+     * @pre nodeIdentity_ is populated (needed for the service_instance_id
+     * fallback), metricsRegistry_ is constructed, and overlay_ (and the other
+     * subsystems read by observable-gauge callbacks) are constructed.
+     */
+    void
+    startTelemetry();
+
     std::shared_ptr<Ledger>
     getLastFullLedger();
 
@@ -1347,7 +1370,8 @@ ApplicationImp::setup(boost::program_options::variables_map const& cmdline)
         telemetry_->setServiceInstanceId(toBase58(TokenType::NodePublic, nodeIdentity_->first));
 
     // Create the OTel MetricsRegistry for gap-fill metrics (counters,
-    // histograms, observable gauges).  It is started later in start().
+    // histograms, observable gauges).  It is started later, just before the
+    // [rpc_startup] loop (see startTelemetry()).
     metricsRegistry_ = std::make_unique<telemetry::MetricsRegistry>(
         telemetry_->isEnabled(), *this, logs_->journal("MetricsRegistry"));
 
@@ -1492,6 +1516,16 @@ ApplicationImp::setup(boost::program_options::variables_map const& cmdline)
         JLOG(journal_.warn()) << "*** standalone signing solution as soon as possible.";
     }
 
+    // Start telemetry and metrics now — before the [rpc_startup] loop below —
+    // so the OTel meter is live before any metric-emitting code runs. Startup
+    // RPCs invoke PerfLog instrumentation that records a call-site metric; a
+    // metric macro caches its instrument on first use, so a first use before
+    // the meter exists would latch null for the process lifetime. Placed here,
+    // after overlay_ and the other subsystems the observable-gauge callbacks
+    // read are constructed, so the metrics reader thread never observes a
+    // half-built application.
+    startTelemetry();
+
     //
     // Execute start up rpc commands.
     //
@@ -1563,6 +1597,12 @@ ApplicationImp::start(bool withTimers)
 
     ledgerCleaner_->start();
     perfLog_->start();
+}
+
+void
+ApplicationImp::startTelemetry()
+{
+    // Start tracing first so subsequent startup/early activity can be traced.
     telemetry_->start();
 
     // Start the metrics pipeline after telemetry; the endpoint uses the