feat(telemetry): add Phase 4 consensus tracing with SpanGuard API

Instrument the consensus subsystem with OpenTelemetry spans covering
the full round lifecycle: round start, establish phase, proposal send,
ledger close, position updates, consensus check, accept, validation
send, and mode changes.

Key design choices adapted from the original Phase 4 implementation
to the new SpanGuard factory pattern introduced in Phase 3:

- Add SpanGuard::hashSpan() for category-gated hash-derived trace IDs
  (consensus round spans share trace_id across validators via ledger hash)
- Add SpanGuard::addEvent() overload with key-value attribute pairs
  (used for dispute.resolve events during position updates)
- Add ConsensusSpanNames.h with compile-time span name constants
  following the colocated *SpanNames.h pattern from Phase 3
- Add consensusTraceStrategy config option ("deterministic"/"attribute")
  for cross-node trace correlation strategy selection
- Use SpanGuard::linkedSpan() for follows-from relationships between
  consecutive rounds and cross-thread validation spans
- Use SpanGuard::captureContext() for thread-safe context propagation
  from consensus thread to jtACCEPT worker thread

Spans produced: consensus.round, consensus.proposal.send,
consensus.ledger_close, consensus.establish, consensus.update_positions,
consensus.check, consensus.accept, consensus.accept.apply,
consensus.validation.send, consensus.mode_change

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

include/xrpl/telemetry/SpanGuard.h

@@ -120,8 +120,10 @@
 #include <array>
 #include <cstdint>
 #include <exception>
+#include <initializer_list>
 #include <memory>
 #include <string_view>
+#include <utility>
 
 namespace xrpl::telemetry {
 
@@ -153,6 +155,11 @@ struct TraceBytes
     bool valid{false};
 };
 
+/** Key-value pair for span event attributes.
+    Used by addEvent(name, attrs) to attach structured metadata to events.
+*/
+using EventAttribute = std::pair<std::string_view, std::string_view>;
+
 /** Opaque wrapper for an OTel context snapshot.
 
     Used to propagate trace context across threads. Created by
@@ -362,6 +369,14 @@ public:
     void
     addEvent(std::string_view name);
 
+    /** Add a named event with key-value attributes to the span's timeline.
+        No-op on a null guard.
+        @param name   Event name.
+        @param attrs  Attribute pairs (all string_view for simplicity).
+    */
+    void
+    addEvent(std::string_view name, std::initializer_list<EventAttribute> attrs);
+
     /** Record an exception as a span event following OTel semantic
         conventions, and mark the span status as error.
         No-op on a null guard.
@@ -491,6 +506,10 @@ public:
     {
     }
     void
+    addEvent(std::string_view, std::initializer_list<EventAttribute>)
+    {
+    }
+    void
     recordException(std::exception const&)
     {
     }

include/xrpl/telemetry/Telemetry.h

@@ -187,6 +187,13 @@ public:
 
         /** Enable tracing for ledger close/accept. */
         bool traceLedger = true;
+
+        /** Strategy for cross-node consensus trace correlation.
+            "deterministic" — derive trace_id from ledger hash so all
+            validators in the same round share the same trace_id.
+            "attribute" — random trace_id, correlate via ledger_id attribute.
+        */
+        std::string consensusTraceStrategy = "deterministic";
     };
 
     virtual ~Telemetry() = default;
@@ -244,6 +251,10 @@ public:
     [[nodiscard]] virtual bool
     shouldTraceLedger() const = 0;
 
+    /** @return The configured consensus trace correlation strategy. */
+    virtual std::string const&
+    getConsensusTraceStrategy() const = 0;
+
 #ifdef XRPL_ENABLE_TELEMETRY
     /** Get or create a named tracer instance.