fix shards computation when there is no metrics yet

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+
+- Manage prometheus-agent configs
+
 ## [0.0.2] - 2024-04-08
 
 ### Fixed

pkg/monitoring/mimir/querier/querier.go

@@ -11,6 +11,13 @@ import (
 	"github.com/prometheus/common/model"
 )
 
+var (
+	ErrorNoTimeSeries                 = errors.New("no time series found")
+	ErrorFailedToConvertValueToVector = errors.New("failed to convert value to vector")
+	ErrorMoreThanOneTimeSeriesFound   = errors.New("more than one time series found")
+	ErrorFailedToGetTimeSeries        = errors.New("failed to get time series")
+)
+
 // QueryTSDBHeadSeries performs an instant query against Mimir.
 func QueryTSDBHeadSeries(ctx context.Context, clusterName string) (float64, error) {
 	config := api.Config{
@@ -38,16 +45,16 @@ func QueryTSDBHeadSeries(ctx context.Context, clusterName string) (float64, erro
 	case model.ValVector:
 		vector, ok := val.(model.Vector)
 		if !ok {
-			return 0, errors.New("failed to convert value to vector")
+			return 0, ErrorFailedToConvertValueToVector
 		}
 		if len(vector) == 0 {
-			return 0, errors.New("no time series found")
+			return 0, ErrorNoTimeSeries
 		}
 		if len(vector) > 1 {
-			return 0, errors.New("more than one time series found")
+			return 0, ErrorMoreThanOneTimeSeriesFound
 		}
 		return float64(vector[0].Value), nil
 	default:
-		return 0, errors.New("failed to get current number of time series")
+		return 0, ErrorFailedToGetTimeSeries
 	}
 }

pkg/monitoring/prometheusagent/config.go

@@ -99,9 +99,10 @@ func getServicePriority(cluster *clusterv1.Cluster) string {
 func getShardsCountForCluster(ctx context.Context, cluster *clusterv1.Cluster, currentShardCount int) (int, error) {
 	headSeries, err := querier.QueryTSDBHeadSeries(ctx, cluster.Name)
 	if err != nil {
-		// Verify that Prometheus is accessible. If not, return the default number of shards.
+		// If Prometheus is not accessible (DNSError), or if we don't have any data yet (ErrNoTimeSeries)
+		// Then, return the default number of shards.
 		var dnsError *net.DNSError
-		if errors.As(err, &dnsError) {
+		if errors.As(err, &dnsError) || errors.Is(err, querier.ErrorNoTimeSeries) {
 			return shards.ComputeShards(currentShardCount, defaultShards), nil
 		}
 		return 0, errors.WithStack(err)