Fix std::bad_weak_ptr crash in OTelGaugeImpl constructor

Remove illegal shared_from_this() call from OTelGaugeImpl constructor.
The shared_ptr control block is not yet associated with the object during
construction, causing std::bad_weak_ptr when [insight] server=otel is
configured. The weakSelf variable was dead code — never used — since the
callback captures `this` directly via void* state. The raw pointer is safe
because RemoveCallback() is called in the destructor.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/libxrpl/beast/insight/OTelCollector.cpp

@@ -562,8 +562,8 @@ OTelGaugeImpl::OTelGaugeImpl(
 {
     // Register the async callback that the SDK calls during collection.
     // The callback reads the atomic value and reports it to the observer.
-    // We capture a weak_ptr to avoid preventing destruction.
-    auto weakSelf = std::weak_ptr<GaugeImpl>(shared_from_this());
+    // The raw `this` pointer is safe here because RemoveCallback() is
+    // called in the destructor before `this` becomes invalid.
     m_gauge->AddCallback(
         [](opentelemetry::metrics::ObserverResult result, void* state) {
             auto* self = static_cast<OTelGaugeImpl*>(state);