[Spanmetrics] - Add exemplars to Sum metrics (#28671)

**Description:** 
We don't have exemplars added to Sum metrics right now. This PR provides
an enhancement to add exemplars to Sum metrics in Spanmetrics connector


**Testing:** 
Added unit tests and also tested it in our local environment.

.chloggen/span-metrics-add-exemplars-to-sum-metric.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: spanmetricsconnector
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Add exemplars to sum metric
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [27451]
+
+# (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: []

connector/spanmetricsconnector/connector.go

@@ -337,6 +337,9 @@ func (p *connectorImp) aggregateMetrics(traces ptrace.Traces) {
 				}
 				// aggregate sums metrics
 				s := sums.GetOrCreate(key, attributes)
+				if p.config.Exemplars.Enabled && !span.TraceID().IsEmpty() {
+					s.AddExemplar(span.TraceID(), span.SpanID(), duration)
+				}
 				s.Add(1)
 
 				// aggregate events metrics

connector/spanmetricsconnector/connector_test.go

@@ -1318,3 +1318,34 @@ func TestSpanMetrics_Events(t *testing.T) {
 		})
 	}
 }
+func TestExemplarsForSumMetrics(t *testing.T) {
+	mcon := consumertest.NewNop()
+	p := newConnectorImp(t, mcon, stringp("defaultNullValue"), explicitHistogramsConfig, enabledExemplarsConfig, cumulative, zaptest.NewLogger(t), nil)
+	traces := buildSampleTrace()
+
+	// Test
+	ctx := metadata.NewIncomingContext(context.Background(), nil)
+
+	err := p.ConsumeTraces(ctx, traces)
+	require.NoError(t, err)
+	metrics := p.buildMetrics()
+
+	for i := 0; i < metrics.ResourceMetrics().Len(); i++ {
+		rm := metrics.ResourceMetrics().At(i)
+		ism := rm.ScopeMetrics()
+		// Checking all metrics, naming notice: ilmC/mC - C here is for Counter.
+		for ilmC := 0; ilmC < ism.Len(); ilmC++ {
+			m := ism.At(ilmC).Metrics()
+			for mC := 0; mC < m.Len(); mC++ {
+				metric := m.At(mC)
+				if metric.Type() == pmetric.MetricTypeSum {
+					dps := metric.Sum().DataPoints()
+					for dpi := 0; dpi < dps.Len(); dpi++ {
+						dp := dps.At(dpi)
+						assert.Greater(t, dp.Exemplars().Len(), 0)
+					}
+				}
+			}
+		}
+	}
+}

connector/spanmetricsconnector/internal/metrics/metrics.go

@@ -232,6 +232,7 @@ func (h *exponentialHistogram) AddExemplar(traceID pcommon.TraceID, spanID pcomm
 type Sum struct {
 	attributes pcommon.Map
 	count      uint64
+	exemplars  pmetric.ExemplarSlice
 }
 
 func (s *Sum) Add(value uint64) {
@@ -251,12 +252,20 @@ func (m *SumMetrics) GetOrCreate(key Key, attributes pcommon.Map) *Sum {
 	if !ok {
 		s = &Sum{
 			attributes: attributes,
+			exemplars:  pmetric.NewExemplarSlice(),
 		}
 		m.metrics[key] = s
 	}
 	return s
 }
 
+func (s *Sum) AddExemplar(traceID pcommon.TraceID, spanID pcommon.SpanID, value float64) {
+	e := s.exemplars.AppendEmpty()
+	e.SetTraceID(traceID)
+	e.SetSpanID(spanID)
+	e.SetDoubleValue(value)
+}
+
 func (m *SumMetrics) BuildMetrics(
 	metric pmetric.Metric,
 	start pcommon.Timestamp,
@@ -273,6 +282,10 @@ func (m *SumMetrics) BuildMetrics(
 		dp.SetStartTimestamp(start)
 		dp.SetTimestamp(timestamp)
 		dp.SetIntValue(int64(s.count))
+		for i := 0; i < s.exemplars.Len(); i++ {
+			s.exemplars.At(i).SetTimestamp(timestamp)
+		}
+		s.exemplars.CopyTo(dp.Exemplars())
 		s.attributes.CopyTo(dp.Attributes())
 	}
 }