This test should be written as a google/benchmark test (see this wiki page and chunk_manager_refresh_bm.cpp as an example).

Outside of the

for (auto keepRunning : state) { ... }

loop, the benchmark should construct a CatalogCache that will return a ChunkManager for the temporary resharding collection which has 60 chunks (to match the number of chunks in ReshardCollectionMixed.yml). It'll additionally need to call CollectionShardingRuntime::setFilteringMetadata() for the source collection with a CollectionMetadata that has reshardingFields present and in a state such that getReshardingKeyIfShouldForwardOps() would return the new shard key pattern.

Within the

for (auto keepRunning : state) { ... }

loop, the benchmark should construct a ReshardingDonorWriteRouter instance and call ReshardingDonorWriteRouter::getDestinedRecipient() for a fixed document. The document should be constructed outside of the loop so that the BSONObjBuilder for it isn't counted in the benchmark.