[rank] adds background job that maps and counts references to definitions (sourcegraph#48004)

Depends on https://github.com/sourcegraph/sourcegraph/pull/48002,
https://github.com/sourcegraph/sourcegraph/pull/47997

Adds background job that maps references to definitions and counts
references per path.

## Test plan
Tested manually.
<!-- All pull requests REQUIRE a test plan:
https://docs.sourcegraph.com/dev/background-information/testing_principles
-->

Author: cesrjimenez

enterprise/internal/codeintel/uploads/init.go

@@ -77,6 +77,7 @@ func NewService(
 
 var (
 	bucketName                   = env.Get("CODEINTEL_UPLOADS_RANKING_BUCKET", "lsif-pagerank-experiments", "The GCS bucket.")
+	rankingMapReduceBatchSize    = env.MustGetInt("CODEINTEL_UPLOADS_MAP_REDUCE_RANKING_BATCH_SIZE", 10000, "How many references, definitions, and path counts to map and reduce at once.")
 	rankingGraphKey              = env.Get("CODEINTEL_UPLOADS_RANKING_GRAPH_KEY", "dev", "An identifier of the graph export. Change to start a new export in the configured bucket.")
 	rankingGraphBatchSize        = env.MustGetInt("CODEINTEL_UPLOADS_RANKING_GRAPH_BATCH_SIZE", 16, "How many uploads to process at once.")
 	rankingGraphDeleteBatchSize  = env.MustGetInt("CODEINTEL_UPLOADS_RANKING_GRAPH_DELETE_BATCH_SIZE", 32, "How many stale uploads to delete at once.")
@@ -208,5 +209,12 @@ func NewGraphExporters(observationCtx *observation.Context, uploadSvc *Service)
 			ConfigExportInst.RankingBatchSize,
 			ConfigExportInst.RankingJobsEnabled,
 		),
+		background.NewRankingGraphMapper(
+			observationCtx,
+			uploadSvc,
+			ConfigExportInst.NumRankingRoutines,
+			ConfigExportInst.RankingInterval,
+			ConfigExportInst.RankingJobsEnabled,
+		),
 	}
 }

enterprise/internal/codeintel/uploads/internal/background/iface.go

@@ -21,6 +21,7 @@ import (
 
 type UploadService interface {
 	ExportRankingGraph(ctx context.Context, numRankingRoutines int, numBatchSize int, rankingJobEnabled bool) error
+	MapRankingGraph(ctx context.Context, numRankingRoutines int, rankingJobEnabled bool) error
 	VacuumRankingGraph(ctx context.Context) error
 }
 

enterprise/internal/codeintel/uploads/internal/background/job_graph_exporter.go

@@ -33,3 +33,22 @@ func NewRankingGraphExporter(
 			return nil
 		}))
 }
+
+func NewRankingGraphMapper(
+	observationCtx *observation.Context,
+	uploadsService UploadService,
+	numRankingRoutines int,
+	interval time.Duration,
+	rankingJobEnabled bool,
+) goroutine.BackgroundRoutine {
+	return goroutine.NewPeriodicGoroutine(
+		context.Background(),
+		"rank.graph-mapper", "maps definitions and references data to path_counts_inputs table in store",
+		interval,
+		goroutine.HandlerFunc(func(ctx context.Context) error {
+			if err := uploadsService.MapRankingGraph(ctx, numRankingRoutines, rankingJobEnabled); err != nil {
+				return err
+			}
+			return nil
+		}))
+}

enterprise/internal/codeintel/uploads/observability.go

@@ -64,6 +64,7 @@ type operations struct {
 
 	// Ranking
 	exportRankingGraph *observation.Operation
+	mapRankingGraph    *observation.Operation
 
 	numUploadsRead         prometheus.Counter
 	numBytesUploaded       prometheus.Counter
@@ -181,6 +182,7 @@ func newOperations(observationCtx *observation.Context) *operations {
 
 		// Ranking
 		exportRankingGraph: op("ExportRankingGraph"),
+		mapRankingGraph:    op("MapRankingGraph"),
 
 		numUploadsRead:         numUploadsRead,
 		numBytesUploaded:       numBytesUploaded,

enterprise/internal/codeintel/uploads/ranking.go

@@ -180,6 +180,21 @@ func (s *Service) VacuumRankingGraph(ctx context.Context) error {
 	return nil
 }
 
+func (s *Service) MapRankingGraph(ctx context.Context, numRankingRoutines int, rankingJobEnabled bool) (err error) {
+	ctx, _, endObservation := s.operations.mapRankingGraph.With(ctx, &err, observation.Args{})
+	defer endObservation(1, observation.Args{})
+
+	if !rankingJobEnabled {
+		return nil
+	}
+
+	if err := s.store.InsertPathCountInputs(ctx, rankingGraphKey, rankingMapReduceBatchSize); err != nil {
+		return err
+	}
+
+	return nil
+}
+
 const maxBytesPerObject = 1024 * 1024 * 1024 // 1GB
 
 func (s *Service) serializeAndPersistRankingGraphForUpload(