Log querier bucket index updates (#9912)

Adds logging of querier bucket index updates so we can inspect exactly which bucket index version/timestamp a particular querier is working off of.

pkg/storage/tsdb/bucketindex/loader.go

@@ -208,6 +208,12 @@ func (l *Loader) updateCachedIndex(ctx context.Context, userID string) {
 	// is when a tenant has rules configured but hasn't started remote writing yet. Rules will be evaluated and
 	// bucket index loaded by the ruler.
 	l.indexesMx.Lock()
+	userIdx := l.indexes[userID]
+	if idx != nil {
+		if userIdx == nil || (userIdx.index != nil && userIdx.index.UpdatedAt != idx.UpdatedAt) {
+			level.Debug(l.logger).Log("msg", "loaded bucket index", "user", userID, "updatedAt", idx.UpdatedAt)
+		}
+	}
 	l.indexes[userID].index = idx
 	l.indexes[userID].err = err
 	l.indexes[userID].setUpdatedAt(startTime)