[exporter/elasticsearch] Emit _doc_count for metric documents in OTel mode when data point attribute _doc_count is true (open-telemetry#35348)

**Description:** <Describe what has changed.>
<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
Emit _doc_count for metric documents in OTel mode when data point
attribute _doc_count is true

**Link to tracking Issue:** <Issue number if applicable>

**Testing:** <Describe what testing was performed and which tests were
added.>

**Documentation:** <Describe the documentation added.>

Author: carsonip

.chloggen/elasticsearchexporter_otel-mode-doc-count.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: elasticsearchexporter
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Emit _doc_count for metric documents in OTel mode when data point attribute _doc_count is true
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [35348]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [user]

exporter/elasticsearchexporter/exporter_test.go

@@ -947,6 +947,53 @@ func TestExporterMetrics(t *testing.T) {
 		assert.Equal(t, `{"some.resource.attribute":["foo","bar"]}`, gjson.GetBytes(doc, `resource.attributes`).Raw)
 	})
 
+	t.Run("otel mode _doc_count", func(t *testing.T) {
+		rec := newBulkRecorder()
+		server := newESTestServer(t, func(docs []itemRequest) ([]itemResponse, error) {
+			rec.Record(docs)
+			return itemsAllOK(docs)
+		})
+
+		exporter := newTestMetricsExporter(t, server.URL, func(cfg *Config) {
+			cfg.Mapping.Mode = "otel"
+		})
+
+		metrics := pmetric.NewMetrics()
+		resourceMetric := metrics.ResourceMetrics().AppendEmpty()
+		scopeMetric := resourceMetric.ScopeMetrics().AppendEmpty()
+
+		sumMetric := scopeMetric.Metrics().AppendEmpty()
+		sumMetric.SetName("sum")
+		sumDP := sumMetric.SetEmptySum().DataPoints().AppendEmpty()
+		sumDP.SetIntValue(0)
+
+		summaryMetric := scopeMetric.Metrics().AppendEmpty()
+		summaryMetric.SetName("summary")
+		summaryDP := summaryMetric.SetEmptySummary().DataPoints().AppendEmpty()
+		summaryDP.SetSum(1)
+		summaryDP.SetCount(10)
+		fillAttributeMap(summaryDP.Attributes(), map[string]any{
+			"_doc_count": true,
+		})
+
+		mustSendMetrics(t, exporter, metrics)
+
+		rec.WaitItems(2)
+		expected := []itemRequest{
+			{
+				Action:   []byte(`{"create":{"_index":"metrics-generic.otel-default","dynamic_templates":{"metrics.summary":"summary_metrics"}}}`),
+				Document: []byte(`{"@timestamp":"1970-01-01T00:00:00.000000000Z","_doc_count":10,"attributes":{"_doc_count":true},"data_stream":{"dataset":"generic.otel","namespace":"default","type":"metrics"},"metrics":{"summary":{"sum":1.0,"value_count":10}},"resource":{"dropped_attributes_count":0},"scope":{"dropped_attributes_count":0}}`),
+			},
+			{
+				Action:   []byte(`{"create":{"_index":"metrics-generic.otel-default","dynamic_templates":{"metrics.sum":"gauge_long"}}}`),
+				Document: []byte(`{"@timestamp":"1970-01-01T00:00:00.000000000Z","data_stream":{"dataset":"generic.otel","namespace":"default","type":"metrics"},"metrics":{"sum":0},"resource":{"dropped_attributes_count":0},"scope":{"dropped_attributes_count":0}}`),
+			},
+		}
+
+		assertItemsEqual(t, expected, rec.Items(), false)
+
+	})
+
 	t.Run("publish summary", func(t *testing.T) {
 		rec := newBulkRecorder()
 		server := newESTestServer(t, func(docs []itemRequest) ([]itemResponse, error) {

exporter/elasticsearchexporter/model.go

@@ -89,6 +89,7 @@ type dataPoint interface {
 	Attributes() pcommon.Map
 	Value() (pcommon.Value, error)
 	DynamicTemplate(pmetric.Metric) string
+	DocCount() uint64
 }
 
 const (
@@ -284,6 +285,7 @@ func (m *encodeModel) upsertMetricDataPointValueOTelMode(documents map[uint32]ob
 	if err != nil {
 		return err
 	}
+
 	// documents is per-resource. Therefore, there is no need to hash resource attributes
 	hash := metricOTelHash(dp, scope.Attributes(), metric.Unit())
 	var (
@@ -302,6 +304,12 @@ func (m *encodeModel) upsertMetricDataPointValueOTelMode(documents map[uint32]ob
 		m.encodeScopeOTelMode(&document, scope, scopeSchemaURL)
 	}
 
+	// Emit _doc_count if data point contains attribute _doc_count: true
+	if val, ok := dp.Attributes().Get("_doc_count"); ok && val.Bool() {
+		docCount := dp.DocCount()
+		document.AddInt("_doc_count", int64(docCount))
+	}
+
 	switch value.Type() {
 	case pcommon.ValueTypeMap:
 		m := pcommon.NewMap()
@@ -340,6 +348,10 @@ func (dp summaryDataPoint) DynamicTemplate(_ pmetric.Metric) string {
 	return "summary_metrics"
 }
 
+func (dp summaryDataPoint) DocCount() uint64 {
+	return dp.Count()
+}
+
 type exponentialHistogramDataPoint struct {
 	pmetric.ExponentialHistogramDataPoint
 }
@@ -367,6 +379,10 @@ func (dp exponentialHistogramDataPoint) DynamicTemplate(_ pmetric.Metric) string
 	return "histogram"
 }
 
+func (dp exponentialHistogramDataPoint) DocCount() uint64 {
+	return dp.Count()
+}
+
 type histogramDataPoint struct {
 	pmetric.HistogramDataPoint
 }
@@ -379,6 +395,10 @@ func (dp histogramDataPoint) DynamicTemplate(_ pmetric.Metric) string {
 	return "histogram"
 }
 
+func (dp histogramDataPoint) DocCount() uint64 {
+	return dp.HistogramDataPoint.Count()
+}
+
 func histogramToValue(dp pmetric.HistogramDataPoint) (pcommon.Value, error) {
 	// Histogram conversion function is from
 	// https://github.com/elastic/apm-data/blob/3b28495c3cbdc0902983134276eb114231730249/input/otlp/metrics.go#L277
@@ -475,6 +495,10 @@ func (dp numberDataPoint) DynamicTemplate(metric pmetric.Metric) string {
 	return ""
 }
 
+func (dp numberDataPoint) DocCount() uint64 {
+	return 1
+}
+
 var errInvalidNumberDataPoint = errors.New("invalid number data point")
 
 func (m *encodeModel) encodeResourceOTelMode(document *objmodel.Document, resource pcommon.Resource, resourceSchemaURL string) {

exporter/elasticsearchexporter/utils_test.go

@@ -288,6 +288,8 @@ func fillAttributeMap(attrs pcommon.Map, m map[string]any) {
 	attrs.EnsureCapacity(len(m))
 	for k, v := range m {
 		switch vv := v.(type) {
+		case bool:
+			attrs.PutBool(k, vv)
 		case string:
 			attrs.PutStr(k, vv)
 		case []string: