[receiver/prometheusremotewrite] skip emitting empty metrics (open-telemetry#44149)

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

We should ideally empty a metric only if it has some datapoints in it.
Right now, we can emit empty metric if the `target_info` arrives first
and normal metrics arrives in subsequent request.

This PR adds a simple check to skip `ConsumeMetrics` if metric count is
0.

<!--Describe what testing was performed and which tests were added.-->
#### Testing

Updated tests

---------

Co-authored-by: Yang Song <[email protected]>

Author: 2 people

.chloggen/emit-nonempty-metrics-opt.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. receiver/filelog)
+component: receiver/prometheusremotewrite
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Skip emitting empty metrics.
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [44149]
+
+# (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: []

receiver/prometheusremotewritereceiver/receiver.go

@@ -194,6 +194,10 @@ func (prw *prometheusRemoteWriteReceiver) handlePRW(w http.ResponseWriter, req *
 
 	w.WriteHeader(http.StatusNoContent)
 
+	// Return if metric count is 0.
+	if m.MetricCount() == 0 {
+		return
+	}
 	obsrecvCtx := prw.obsrecv.StartMetricsOp(req.Context())
 	err = prw.nextConsumer.ConsumeMetrics(req.Context(), m)
 	if err != nil {

receiver/prometheusremotewritereceiver/receiver_test.go

@@ -1509,21 +1509,6 @@ func TestTargetInfoWithMultipleRequests(t *testing.T) {
 		attrs.PutStr("cloud_provider", "gcp")
 		attrs.PutStr("region", "us-central1")
 
-		return metrics
-	}()
-
-	// Using the same expected metrics for both tests, because we are just checking if the order of the requests changes the result.
-	expectedIndex1 := func() pmetric.Metrics {
-		metrics := pmetric.NewMetrics()
-		rm := metrics.ResourceMetrics().AppendEmpty()
-		attrs := rm.Resource().Attributes()
-		attrs.PutStr("service.namespace", "production")
-		attrs.PutStr("service.name", "service_a")
-		attrs.PutStr("service.instance.id", "host1")
-		attrs.PutStr("machine_type", "n1-standard-1")
-		attrs.PutStr("cloud_provider", "gcp")
-		attrs.PutStr("region", "us-central1")
-
 		sm := rm.ScopeMetrics().AppendEmpty()
 		sm.Scope().SetName("OpenTelemetry Collector")
 		sm.Scope().SetVersion("latest")
@@ -1568,11 +1553,10 @@ func TestTargetInfoWithMultipleRequests(t *testing.T) {
 				assert.NoError(t, err)
 				assert.Equal(t, http.StatusNoContent, resp.StatusCode, string(body))
 			}
-
-			// index 0 is related to the target_info metric that is the first request.
+			// Only one metric should be emitted
+			assert.Len(t, mockConsumer.metrics, 1)
+			// index 0 is related to the join between the target_info and the normal metric.
 			assert.NoError(t, pmetrictest.CompareMetrics(expectedIndex0, mockConsumer.metrics[0]))
-			// index 1 is related to the join between the target_info and the normal metric.
-			assert.NoError(t, pmetrictest.CompareMetrics(expectedIndex1, mockConsumer.metrics[1]))
 		})
 	}
 }
@@ -1768,11 +1752,11 @@ func TestLRUCacheResourceMetrics(t *testing.T) {
 	}
 
 	// As target_info and metric1 have the same job/instance, they generate the same end metric: mockConsumer.metrics[0].
-	assert.NoError(t, pmetrictest.CompareMetrics(expectedMetrics1, mockConsumer.metrics[1]))
-	// As metric2 have different job/instance, it generates a different end metric: mockConsumer.metrics[2]. At this point, the cache is full it should evict the target_info metric to store the metric2.
-	assert.NoError(t, pmetrictest.CompareMetrics(expectedMetrics2, mockConsumer.metrics[2]))
+	assert.NoError(t, pmetrictest.CompareMetrics(expectedMetrics1, mockConsumer.metrics[0]))
+	// As metric2 have different job/instance, it generates a different end metric: mockConsumer.metrics[1]. At this point, the cache is full it should evict the target_info metric to store the metric2.
+	assert.NoError(t, pmetrictest.CompareMetrics(expectedMetrics2, mockConsumer.metrics[1]))
 	// As just have 1 slot in the cache, but the cache for metric1 was evicted, this metric1_1 should generate a new resource metric, even having the same job/instance than the metric1.
-	assert.NoError(t, pmetrictest.CompareMetrics(expectedMetrics1_1, mockConsumer.metrics[3]))
+	assert.NoError(t, pmetrictest.CompareMetrics(expectedMetrics1_1, mockConsumer.metrics[2]))
 }
 
 func buildMetaDataMapByID(ms pmetric.Metrics) map[string]map[string]any {