Implement host + C API plumbing to expose a snapshot of SearchIdLookupMetrics through MongoExtensionQueryExecutionContext::get_host_metrics, so the $search extension can tune batch size dynamically based on idLookup success rate. Add appropriate unit and integration tests.

Since get_host_metrics and get_metrics are similar but with different purposes and paths, we should make sure to document the distinction between them in api.h.