performance-metrics: Add util module with shared micro benchmark helpers

These factor out common setup and synchronization patterns used by block
layer micro benchmarks.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

performance-metrics/src/main.rs

@@ -5,6 +5,7 @@
 
 // Custom harness to run performance tests
 mod performance_tests;
+mod util;
 
 use std::process::Command;
 use std::sync::Arc;