add bp2build subevents

Test: verified new events in bp2build_metrics.pb
Bug: 286412441
Change-Id: Ib803c3a3d1a8e7eaae5fb4c01c7769a72f4664ff
diff --git a/bp2build/build_conversion.go b/bp2build/build_conversion.go
index 09a9d04..6f41f37 100644
--- a/bp2build/build_conversion.go
+++ b/bp2build/build_conversion.go
@@ -653,6 +653,8 @@
 }
 
 func GenerateBazelTargets(ctx *CodegenContext, generateFilegroups bool) (conversionResults, []error) {
+	ctx.Context().BeginEvent("GenerateBazelTargets")
+	defer ctx.Context().EndEvent("GenerateBazelTargets")
 	buildFileToTargets := make(map[string]BazelTargets)
 
 	// Simple metrics tracking for bp2build