lib/protoparser/opentelemetry: allow ingesting metrics without resource labels

Some clients may ingest samples via OpenTelemetry protocol without Resource labels.
Previously VictoriaMetrics was silently dropping such samples.

The commit 317834f876 added vm_protoparser_rows_dropped_total{type="opentelemetry",reason="resource_not_set"}
counter for tracking of such dropped samples. See https://github.com/VictoriaMetrics/VictoriaMetrics/pull/5459

It is better from usability PoV to accept such samples instead of dropping them and incrementing the corresponding counter.

docs/CHANGELOG.md

@@ -30,8 +30,8 @@ The sandbox cluster installation is running under the constant load generated by
 
 * FEATURE: [vmauth](https://docs.victoriametrics.com/vmauth.html): add ability to proxy incoming requests to different backends based on the requested host via `src_hosts` option at `url_map`. See [these docs](https://docs.victoriametrics.com/vmauth.html#generic-http-proxy-for-different-backends).
 
+* BUGFIX: `vminsert`: properly accept samples via [OpenTelemetry data ingestion protocol](https://docs.victoriametrics.com/#sending-data-via-opentelemetry) when these samples have no [resource attributes](https://opentelemetry.io/docs/instrumentation/go/resources/). Previously such samples were silently skipped.
 * BUGFIX: `vmstorage`: added missing `-inmemoryDataFlushInterval` command-line flag, which was missing in [VictoriaMetrics cluster](https://docs.victoriametrics.com/Cluster-VictoriaMetrics.html) after implementing [this feature](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3337) in [v1.85.0](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v1.85.0).
-
 * BUGFIX: [vmalert](https://docs.victoriametrics.com/vmalert.html): check `-external.url` schema when starting vmalert, must be `http` or `https`. Before, alertmanager could reject alert notifications if `-external.url` contained no or wrong schema.
 * BUGFIX: [MetricsQL](https://docs.victoriametrics.com/MetricsQL.html): properly handle queries, which wrap [rollup functions](https://docs.victoriametrics.com/MetricsQL.html#rollup-functions) with multiple arguments without explicitly specified lookbehind window in square brackets into [aggregate functions](https://docs.victoriametrics.com/MetricsQL.html#aggregate-functions). For example, `sum(quantile_over_time(0.5, process_resident_memory_bytes))` was resulting to `expecting at least 2 args to ...; got 1 args' error. Thanks to @atykhyy for [the pull request](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/5414).
 * BUGFIX: [vmctl](https://docs.victoriametrics.com/vmctl.html): retry on import errors in `vm-native` mode. Before, retries happened only on writes into a network connection between source and destination. But errors returned by server after all the data was transmitted were logged, but not retried.  

lib/protoparser/opentelemetry/stream/streamparser.go

@@ -272,12 +272,11 @@ func (wr *writeContext) readAndUnpackRequest(r io.Reader) (*pb.ExportMetricsServ
 
 func (wr *writeContext) parseRequestToTss(req *pb.ExportMetricsServiceRequest) {
 	for _, rm := range req.ResourceMetrics {
-		if rm.Resource == nil {
+		var attributes []*pb.KeyValue
-			// skip metrics without resource part.
+		if rm.Resource != nil {
-			rowsDroppedResourceNotSet.Inc()
+			attributes = rm.Resource.Attributes
-			continue
 		}
-		wr.baseLabels = appendAttributesToPromLabels(wr.baseLabels[:0], rm.Resource.Attributes)
+		wr.baseLabels = appendAttributesToPromLabels(wr.baseLabels[:0], attributes)
 		for _, sc := range rm.ScopeMetrics {
 			wr.appendSamplesFromScopeMetrics(sc)
 		}
@@ -304,5 +303,4 @@ var (
 	rowsDroppedUnsupportedHistogram  = metrics.NewCounter(`vm_protoparser_rows_dropped_total{type="opentelemetry",reason="unsupported_histogram_aggregation"}`)
 	rowsDroppedUnsupportedSum        = metrics.NewCounter(`vm_protoparser_rows_dropped_total{type="opentelemetry",reason="unsupported_sum_aggregation"}`)
 	rowsDroppedUnsupportedMetricType = metrics.NewCounter(`vm_protoparser_rows_dropped_total{type="opentelemetry",reason="unsupported_metric_type"}`)
-	rowsDroppedResourceNotSet        = metrics.NewCounter(`vm_protoparser_rows_dropped_total{type="opentelemetry",reason="resource_not_set"}`)
 )