fix(telemetry): mark macro-only includes IWYU keep in MetricMacros.h

misc-include-cleaner does not expand macros, so it flags MetricsRegistry.h,
ServiceRegistry.h, <functional> and <mutex> as unused even though every
XRPL_METRIC_* macro body uses them ((app).getMetricsRegistry(),
std::once_flag/std::call_once, std::function). Add IWYU pragma: keep, the
same suppression already used across xrpld/ for macro/transitive includes.

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

src/xrpld/telemetry/MetricMacros.h

@@ -101,14 +101,14 @@
  * tasks/metric-macro-plan.md.
  */
 
-#include <xrpld/telemetry/MetricsRegistry.h>
+#include <xrpld/telemetry/MetricsRegistry.h>  // IWYU pragma: keep
 
-#include <xrpl/core/ServiceRegistry.h>
+#include <xrpl/core/ServiceRegistry.h>  // IWYU pragma: keep
 
 #ifdef XRPL_ENABLE_TELEMETRY
 
-#include <functional>
-#include <mutex>
+#include <functional>  // IWYU pragma: keep
+#include <mutex>       // IWYU pragma: keep
 
 #define XRPL_METRIC_COUNTER_INC(app, name, description)                                        \
     do                                                                                         \