runtime: add race annotations to metricsSema

metricsSema protects the metrics map. The map implementation is race
instrumented regardless of which package is it called from.

semacquire/semrelease are not automatically race instrumented, so we can
trigger race false positives without manually annotating our lock
acquire and release.

See similar instrumentation on trace.shutdownSema and reflectOffs.lock.

Fixes #53542.

Change-Id: Ia3fd239ac860e037d09c7cb9c4ad267391e70705
Reviewed-on: https://go-review.googlesource.com/c/go/+/414517
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/runtime/export_test.go

@@ -312,9 +312,9 @@ func ReadMetricsSlow(memStats *MemStats, samplesp unsafe.Pointer, len, cap int)
 
 	// Initialize the metrics beforehand because this could
 	// allocate and skew the stats.
-	semacquire(&metricsSema)
+	metricsLock()
 	initMetrics()
-	semrelease(&metricsSema)
+	metricsUnlock()
 
 	systemstack(func() {
 		// Read memstats first. It's going to flush