[connector/spanmetricsconnector] Initialise new calls_total metrics at 0 (open-telemetry#39052)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description
fix:
open-telemetry#38537

Signed-off-by: Murphy Chen <[email protected]>

Author: Frapschen

.chloggen/spanmetrics-call_total-init-witth-0.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: Initialise new calls_total metrics at 0
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [38537]
+
+# (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]

connector/spanmetricsconnector/connector_test.go

@@ -188,8 +188,13 @@ func verifyConsumeMetricsInput(tb testing.TB, input pmetric.Metrics, expectedTem
 		require.Equal(tb, numDataPoints, callsDps.Len())
 		for dpi := 0; dpi < numDataPoints; dpi++ {
 			dp := callsDps.At(dpi)
+			expectIntValue := numCumulativeConsumptions
+			// this calls init value is 0 for the first Consumption.
+			if numCumulativeConsumptions == 1 {
+				expectIntValue = 0
+			}
 			assert.Equal(tb,
-				int64(numCumulativeConsumptions),
+				int64(expectIntValue),
 				dp.IntValue(),
 				"There should only be one metric per Service/name/kind combination",
 			)

connector/spanmetricsconnector/internal/metrics/metrics.go

@@ -241,6 +241,11 @@ type Sum struct {
 	count            uint64
 	exemplars        pmetric.ExemplarSlice
 	maxExemplarCount *int
+	// isFirst is used to track if this datapoint is new to the Sum. This
+	// is used to ensure that new Sum metrics being with 0, and then are incremented
+	// to the desired value.  This avoids Prometheus throwing away the first
+	// value in the series, due to the transition from null -> x.
+	isFirst bool
 }
 
 func (s *Sum) Add(value uint64) {
@@ -266,6 +271,7 @@ func (m *SumMetrics) GetOrCreate(key Key, attributes pcommon.Map) *Sum {
 			attributes:       attributes,
 			exemplars:        pmetric.NewExemplarSlice(),
 			maxExemplarCount: m.maxExemplarCount,
+			isFirst:          true,
 		}
 		m.metrics[key] = s
 	}
@@ -297,7 +303,12 @@ func (m *SumMetrics) BuildMetrics(
 		dp := dps.AppendEmpty()
 		dp.SetStartTimestamp(startTimestamp(k))
 		dp.SetTimestamp(timestamp)
-		dp.SetIntValue(int64(s.count))
+		if s.isFirst {
+			dp.SetIntValue(0)
+			s.isFirst = false
+		} else {
+			dp.SetIntValue(int64(s.count))
+		}
 		for i := 0; i < s.exemplars.Len(); i++ {
 			s.exemplars.At(i).SetTimestamp(timestamp)
 		}