Improve indexing observability (#108901)

* Add details to tracing spans when creating index.

* Log reason for building index.

* Log reason for building index.

* Remove initialization of labels to avoid unnecessary metrics.

* Track succcessful, failed and skiped index builds. Track index build time for individual index, not all indexes.

* Revert removal of labels initialization.

pkg/storage/unified/testing/benchmark.go

@@ -215,7 +215,7 @@ func runSearchBackendBenchmarkWriteThroughput(ctx context.Context, backend resou
 
 	// Build initial index
 	size := int64(10000) // force the index to be on disk
-	index, err := backend.BuildIndex(ctx, nr, size, 0, nil, func(index resource.ResourceIndex) (int64, error) {
+	index, err := backend.BuildIndex(ctx, nr, size, 0, nil, "benchmark", func(index resource.ResourceIndex) (int64, error) {
 		return 0, nil
 	})
 	if err != nil {