feat(telemetry): add txq expired/dropped counters for queue backpressure

The transaction queue had no metric for demand that leaves or never enters the
queue, so fee-underpayment abandonment and admission-control rejection were
invisible (distinct from jq_trans_overflow, which is the job queue).

Add two synchronous counters via MetricsRegistry:
- xrpld_txq_expired_total — incremented in TxQ::processClosedLedger() for each
  queued transaction removed because its LastLedgerSequence passed (submitters
  who under-bid the escalating fee and were never included).
- xrpld_txq_dropped_total{reason} — incremented in TxQ::apply() at the
  queue-full admission-control returns (reason="queue_full").

Both reach MetricsRegistry via the Application& parameter already passed to
these methods; calls are null-guarded so they no-op when telemetry is disabled.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/xrpld/app/misc/detail/TxQ.cpp

@@ -3,6 +3,7 @@
 #include <xrpld/app/ledger/OpenLedger.h>
 #include <xrpld/app/main/Application.h>
 #include <xrpld/app/misc/detail/TxQSpanNames.h>
+#include <xrpld/telemetry/MetricsRegistry.h>
 
 #include <xrpl/basics/BasicConfig.h>
 #include <xrpl/basics/Log.h>
@@ -1254,6 +1255,8 @@ TxQ::apply(
             JLOG(j_.info()) << "Queue is full, and transaction " << transactionID
                             << " would kick a transaction from the same account (" << account
                             << ") out of the queue.";
+            if (auto* const metrics = app.getMetricsRegistry(); metrics != nullptr)
+                metrics->incrementTxqDropped("queue_full");
             return {telCAN_NOT_QUEUE_FULL, false};
         }
         auto const& endAccount = byAccount_.at(lastRIter->account);
@@ -1297,6 +1300,8 @@ TxQ::apply(
         {
             JLOG(j_.info()) << "Queue is full, and transaction " << transactionID
                             << " fee is lower than end item's account average fee";
+            if (auto* const metrics = app.getMetricsRegistry(); metrics != nullptr)
+                metrics->incrementTxqDropped("queue_full");
             return {telCAN_NOT_QUEUE_FULL, false};
         }
     }
@@ -1366,12 +1371,17 @@ TxQ::processClosedLedger(Application& app, ReadView const& view, bool timeLeap)
         maxSize_ = std::max(snapshot.txnsExpected * setup_.ledgersInQueue, setup_.queueSizeMin);
 
     // Remove any queued candidates whose LastLedgerSequence has gone by.
+    auto* const metrics = app.getMetricsRegistry();
     for (auto candidateIter = byFee_.begin(); candidateIter != byFee_.end();)
     {
         if (candidateIter->lastValid && *candidateIter->lastValid <= ledgerSeq)
         {
             byAccount_.at(candidateIter->account).dropPenalty = true;
             candidateIter = erase(candidateIter);
+            // Count each expired transaction: submitters who under-bid the
+            // escalating fee and were never included before expiry.
+            if (metrics != nullptr)
+                metrics->incrementTxqExpired();
         }
         else
         {

src/xrpld/telemetry/MetricsRegistry.cpp

@@ -240,6 +240,10 @@ MetricsRegistry::start(std::string const& endpoint, std::string const& instanceI
     ledgerHistoryMismatchCounter_ = meter_->CreateUInt64Counter(
         "xrpld_ledger_history_mismatch_total",
         "Total built-vs-validated ledger mismatches by reason");
+    txqExpiredCounter_ = meter_->CreateUInt64Counter(
+        "xrpld_txq_expired_total", "Total transactions expired out of the transaction queue");
+    txqDroppedCounter_ = meter_->CreateUInt64Counter(
+        "xrpld_txq_dropped_total", "Total transactions refused admission to the queue by reason");
     validationAgreementsCounter_ = meter_->CreateUInt64Counter(
         "xrpld_validation_agreements_total", "Total validation agreements");
     validationMissedCounter_ =
@@ -1338,4 +1342,22 @@ MetricsRegistry::incrementLedgerHistoryMismatch(std::string_view reason)
 #endif
 }
 
+void
+MetricsRegistry::incrementTxqExpired()
+{
+#ifdef XRPL_ENABLE_TELEMETRY
+    if (enabled_ && txqExpiredCounter_)
+        txqExpiredCounter_->Add(1);
+#endif
+}
+
+void
+MetricsRegistry::incrementTxqDropped(std::string_view reason)
+{
+#ifdef XRPL_ENABLE_TELEMETRY
+    if (enabled_ && txqDroppedCounter_)
+        txqDroppedCounter_->Add(1, {{"reason", std::string(reason)}});
+#endif
+}
+
 }  // namespace xrpl::telemetry

src/xrpld/telemetry/MetricsRegistry.h

@@ -360,6 +360,23 @@ public:
     void
     incrementLedgerHistoryMismatch(std::string_view reason);
 
+    /** Increment the txq_expired_total counter.
+        Called from TxQ::processClosedLedger() for each queued transaction
+        removed because its LastLedgerSequence has passed — submitters who
+        under-bid the escalating fee and were never included.
+    */
+    void
+    incrementTxqExpired();
+
+    /** Increment the txq_dropped_total{reason} counter.
+        Called from TxQ::apply() when a transaction is refused admission to
+        the queue (e.g. the queue is full). Distinct from expiry (already
+        queued) and from jq_trans_overflow (job queue, not TxQ).
+        @param reason Admission-control rejection cause (e.g. "queue_full").
+    */
+    void
+    incrementTxqDropped(std::string_view reason);
+
     /** Access the validation agreement tracker.
         Used by consensus and ledger hooks to record our validations and
         network validations so the tracker can compute agreement percentages.
@@ -498,6 +515,12 @@ private:
     /// built-vs-validated ledger mismatch.
     opentelemetry::nostd::unique_ptr<opentelemetry::metrics::Counter<uint64_t>>
         ledgerHistoryMismatchCounter_;
+    /// Counter: xrpld_txq_expired_total — incremented per transaction expired out of the
+    /// transaction queue.
+    opentelemetry::nostd::unique_ptr<opentelemetry::metrics::Counter<uint64_t>> txqExpiredCounter_;
+    /// Counter: xrpld_txq_dropped_total{reason} — incremented when a transaction is refused
+    /// admission to the queue.
+    opentelemetry::nostd::unique_ptr<opentelemetry::metrics::Counter<uint64_t>> txqDroppedCounter_;
     /// Counter: xrpld_validation_agreements_total — incremented by ValidationTracker on
     /// agreement.
     opentelemetry::nostd::unique_ptr<opentelemetry::metrics::Counter<uint64_t>>