db: deflake TestMetrics

Fix #4624.

Author: jbowens

data_test.go

@@ -1379,6 +1379,10 @@ func runPopulateCmd(t *testing.T, td *datadriven.TestData, b *Batch) {
 // waitTableStatsInitialLoad waits until the statistics for all files that
 // existed during Open have been loaded; used in tests.
 // The d.mu mutex must be locked while calling this method.
+//
+// TODO(jackson): Consolidate waitTableStatsInitialLoad and waitTableStats. It
+// seems to be possible for loadedInitial to remain indefinitely false in some
+// tests that use waitTableStats today.
 func (d *DB) waitTableStatsInitialLoad() {
 	for !d.mu.tableStats.loadedInitial {
 		d.mu.tableStats.cond.Wait()

metrics_test.go

@@ -341,6 +341,7 @@ func TestMetrics(t *testing.T) {
 			// The asynchronous loading of table stats can change metrics, so
 			// wait for all the tables' stats to be loaded.
 			d.mu.Lock()
+			d.waitTableStatsInitialLoad()
 			d.waitTableStats()
 			d.mu.Unlock()