fix(telemetry): add getMeter override to test mock (originating branch)

getMeter() became a pure virtual on THIS branch (native-metrics), which
makes the TestTelemetry mock abstract and would break a telemetry=ON
build of phase7/phase8 in isolation. The override was previously only on
phase9 (where it was mis-attributed); relocate it to phase7 where the
pure virtual is introduced, so every branch from here forward builds.
Mirrors NullTelemetry: an inert meter from a process-wide noop provider.

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

src/tests/libxrpl/telemetry/SpanGuardScope.cpp

@@ -19,6 +19,9 @@
 #include <opentelemetry/context/context.h>
 #include <opentelemetry/exporters/memory/in_memory_span_data.h>
 #include <opentelemetry/exporters/memory/in_memory_span_exporter_factory.h>
+#include <opentelemetry/metrics/meter.h>
+#include <opentelemetry/metrics/meter_provider.h>
+#include <opentelemetry/metrics/noop.h>
 #include <opentelemetry/nostd/shared_ptr.h>
 #include <opentelemetry/sdk/resource/resource.h>
 #include <opentelemetry/sdk/trace/samplers/always_on_factory.h>
@@ -152,6 +155,19 @@ public:
         return provider_->GetTracer(std::string(name));
     }
 
+    /**
+     * @return A meter from a noop provider; the scope tests exercise only
+     * tracing, so metrics instruments are inert.
+     */
+    opentelemetry::nostd::shared_ptr<opentelemetry::metrics::Meter>
+    getMeter(std::string_view name) override
+    {
+        static auto noopProvider =
+            opentelemetry::nostd::shared_ptr<opentelemetry::metrics::MeterProvider>(
+                new opentelemetry::metrics::NoopMeterProvider());
+        return noopProvider->GetMeter(std::string(name), std::string(kMeterVersion));
+    }
+
     opentelemetry::nostd::shared_ptr<opentelemetry::trace::Span>
     startSpan(std::string_view name, opentelemetry::trace::SpanKind kind) override
     {