Merge branch 'pratik/otel-phase8-log-correlation' into pratik/otel-phase9-metric-gap-fill

Conflict resolutions:

- docker/telemetry/xrpld-telemetry.cfg: relocation conflict. phase-9 had
  already moved [insight] to the end of the file with server=otel, so the
  incoming block was dropped rather than inserted. Keeping both would have
  produced two [insight] sections, which merge last-wins into a single
  effective section, silently reviving the bug this branch just fixed.
  phase-9's per-branch service_instance_id=xrpld-devnet is preserved.

- OpenTelemetryPlan/06-implementation-phases.md: kept both corrections.
  phase-9's "Tempo" is right (no Jaeger anywhere in the stack) and
  phase-8's "active, sampled span" is right: Log.cpp:328 injects only
  when spanCtx.IsValid() && spanCtx.IsSampled().

- OpenTelemetryPlan/09-data-collection-reference.md and
  docs/telemetry-runbook.md: kept phase-9's structured-metadata LogQL.
  The collector's filelog regex_parser already extracts partition,
  severity, trace_id and span_id, so phase-8's inline regexp forms are
  redundant, and a line filter matches the literal text in a message body.

src/libxrpl/basics/Log.cpp

@@ -302,9 +302,9 @@ Logs::format(
     }
 
 #ifdef XRPL_ENABLE_TELEMETRY
-    // Inject OTel trace context when an active span exists on this thread.
-    // Checks the thread-local context value directly to avoid the heap
-    // allocation that GetSpan() performs on the no-span path.
+    // Inject OTel trace context when an active, sampled span exists on this
+    // thread. Checks the thread-local context value directly to avoid the
+    // heap allocation that GetSpan() performs on the no-span path.
     {
         auto context = opentelemetry::context::RuntimeContext::GetCurrent();
         auto spanValue = context.GetValue(opentelemetry::trace::kSpanKey);
@@ -314,7 +314,18 @@ Logs::format(
             auto span = opentelemetry::nostd::get<
                 opentelemetry::nostd::shared_ptr<opentelemetry::trace::Span>>(spanValue);
             auto spanCtx = span->GetContext();
-            if (spanCtx.IsValid())
+            // Require the sampled flag as well as a valid context. A dropped
+            // span still carries its parent's ids, so a valid context does
+            // not imply the span reaches the backend. An unsampled remote
+            // parent arrives either because an upstream node propagated
+            // sampled=0, or because a peer omitted trace_flags entirely and
+            // it defaults to 0 (TraceContextPropagator, TxTracing,
+            // ConsensusReceiveTracing). Either way the ParentBasedSampler
+            // drops the local span, while the tracer still returns a no-op
+            // span with a valid context.
+            // Logging those ids would advertise a trace that was never
+            // exported, leaving the log-to-trace link resolving to nothing.
+            if (spanCtx.IsValid() && spanCtx.IsSampled())
             {
                 // Hex widths of a W3C trace context: 16-byte trace_id and
                 // 8-byte span_id render to 32 and 16 lowercase hex chars.