Replace SortedStringKeys with SortedKeys

Now that we have generics.

Bug: 193460475
Test: presubmits
Change-Id: I1594fd8feb505175d5c09c03ef397e5ffd5b09cb
diff --git a/bp2build/metrics.go b/bp2build/metrics.go
index 7e29fac..a020650 100644
--- a/bp2build/metrics.go
+++ b/bp2build/metrics.go
@@ -51,7 +51,7 @@
 // Print the codegen metrics to stdout.
 func (metrics *CodegenMetrics) Print() {
 	generatedTargetCount := uint64(0)
-	for _, ruleClass := range android.SortedStringKeys(metrics.serialized.RuleClassCount) {
+	for _, ruleClass := range android.SortedKeys(metrics.serialized.RuleClassCount) {
 		count := metrics.serialized.RuleClassCount[ruleClass]
 		fmt.Printf("[bp2build] %s: %d targets\n", ruleClass, count)
 		generatedTargetCount += count