Add experimental_allow_mempurge flag to benchmark. (#8546)

Summary:
Tiny PR to add the `experimental_allow_mempurge` to the `db_bench` tool (`Mempurge` is the current prototype for memtable garbage collection).
This is useful to benchmark the prototype of this new feature, stress test it and help find new meaningful heuristics for GC.
By default, the flag to allow `mempurge` is set to `false`.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8546

Reviewed By: anand1976

Differential Revision: D29738338

Pulled By: bjlemaire

fbshipit-source-id: 01892883a2f1c714c110718674da05992d6e2dd6

tools/db_bench_tool.cc

@@ -1229,6 +1229,9 @@ DEFINE_bool(
 DEFINE_bool(allow_concurrent_memtable_write, true,
             "Allow multi-writers to update mem tables in parallel.");
 
+DEFINE_bool(experimental_allow_mempurge, false,
+            "Allow memtable garbage collection.");
+
 DEFINE_bool(inplace_update_support,
             ROCKSDB_NAMESPACE::Options().inplace_update_support,
             "Support in-place memtable update for smaller or same-size values");
@@ -4302,6 +4305,7 @@ class Benchmark {
     options.delayed_write_rate = FLAGS_delayed_write_rate;
     options.allow_concurrent_memtable_write =
         FLAGS_allow_concurrent_memtable_write;
+    options.experimental_allow_mempurge = FLAGS_experimental_allow_mempurge;
     options.inplace_update_support = FLAGS_inplace_update_support;
     options.inplace_update_num_locks = FLAGS_inplace_update_num_locks;
     options.enable_write_thread_adaptive_yield =