fix(api): guard allocation tracing RPC behind cfg feature flag

The `get_allocation_tracing_status` RPC handler unconditionally called
`is_allocation_tracing_enabled()`, which only exists behind the
`allocation-tracing` feature gate. This broke builds on feature sets
that lack it (e.g. `default-msvc`). Add cfg guards so the handler
returns `false` when the feature is absent.

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

Author: pront

lib/vector-top/src/metrics.rs

@@ -573,6 +573,11 @@ pub async fn init_components(
 
     #[cfg(feature = "allocation-tracing")]
     {
+        // Allocation tracing is a compile-time + startup-time setting on the
+        // server, so querying once per connection is sufficient. On error
+        // (e.g. older server without this RPC) we default to false, matching
+        // pre-existing behavior. This is re-evaluated on every reconnect via
+        // the retry loop in `subscription()`.
         state.allocation_tracing_active = client
             .get_allocation_tracing_status()
             .await

src/api/grpc/service.rs

@@ -440,7 +440,10 @@ impl observability::Service for ObservabilityService {
         &self,
         _request: Request<GetAllocationTracingStatusRequest>,
     ) -> Result<Response<GetAllocationTracingStatusResponse>, Status> {
+        #[cfg(feature = "allocation-tracing")]
         let enabled = crate::internal_telemetry::allocations::is_allocation_tracing_enabled();
+        #[cfg(not(feature = "allocation-tracing"))]
+        let enabled = false;
         Ok(Response::new(GetAllocationTracingStatusResponse {
             enabled,
         }))