[chore][exporter/datadog] Fix and un-skip flaky integration tests (#49085)

#### Description
- Fix 2 integration tests where payloads were not properly decoded
before (zlib instead of gzip)
- Hard-code a hsotname in test configs to reduce the test run time (from
~25s to ~10s)

#### Authorship

- [x] I, a human, wrote this pull request description myself.

Author: songy23

exporter/datadogexporter/integrationtest/integration_test.go

@@ -12,6 +12,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"maps"
 	"net/http"
 	"strings"
 	"sync"
@@ -442,11 +443,6 @@ func sendTracesComputeTopLevelBySpanKind(t *testing.T, endpoint string) {
 }
 
 func TestIntegrationLogs(t *testing.T) {
-	require.NoError(t, featuregate.GlobalRegistry().Set("exporter.datadogexporter.metricexportserializerclient", false))
-	defer func() {
-		require.NoError(t, featuregate.GlobalRegistry().Set("exporter.datadogexporter.metricexportserializerclient", true))
-	}()
-
 	// 1. Set up mock Datadog server
 	// See also https://github.com/DataDog/datadog-agent/blob/49c16e0d4deab396626238fa1d572b684475a53f/cmd/trace-agent/test/backend.go
 	seriesRec := &testutil.HTTPRequestRecorderWithChan{Pattern: testutil.MetricV2Endpoint, ReqChan: make(chan []byte)}
@@ -499,11 +495,9 @@ func TestIntegrationLogs(t *testing.T) {
 		case <-doneChannel:
 			assert.Len(t, logsData, 5)
 		case metricsBytes := <-seriesRec.ReqChan:
-			var smap seriesSlice
-			gz := getGzipReader(t, metricsBytes)
-			dec := json.NewDecoder(gz)
-			assert.NoError(t, dec.Decode(&smap))
-			for _, s := range smap.Series {
+			newSeries, err := seriesSliceFromAny(t, metricsBytes)
+			require.NoError(t, err)
+			for _, s := range newSeries {
 				if s.Metric == "otelcol_receiver_accepted_log_records" || s.Metric == "otelcol_exporter_sent_log_records" {
 					metricMap.Series = append(metricMap.Series, s)
 				}
@@ -732,11 +726,6 @@ func seriesFromAPIClient(t *testing.T, metricsBytes []byte, expectedMetrics map[
 }
 
 func TestIntegrationInternalMetrics(t *testing.T) {
-	t.Skip("flaky test http://github.com/open-telemetry/opentelemetry-collector-contrib/issues/40056")
-	require.NoError(t, featuregate.GlobalRegistry().Set("exporter.datadogexporter.metricexportserializerclient", false))
-	defer func() {
-		require.NoError(t, featuregate.GlobalRegistry().Set("exporter.datadogexporter.metricexportserializerclient", true))
-	}()
 	expectedMetrics := map[string]struct{}{
 		// Datadog internal metrics on trace and stats writers
 		"datadog.otlp_translator.resources.missing_source": {},
@@ -807,19 +796,82 @@ func testIntegrationInternalMetrics(t *testing.T, expectedMetrics map[string]str
 		case <-tracesRec.ReqChan:
 			// Drain the channel, no need to look into the traces
 		case metricsBytes := <-seriesRec.ReqChan:
-			var metrics seriesSlice
-			gz := getGzipReader(t, metricsBytes)
-			dec := json.NewDecoder(gz)
-			assert.NoError(t, dec.Decode(&metrics))
-			for _, s := range metrics.Series {
-				if _, ok := expectedMetrics[s.Metric]; ok {
-					metricMap[s.Metric] = s
-				}
-			}
+			// The mock server receives two different compression formats:
+			// - gzip JSON (from the DD trace agent's internal metrics reporter, magic bytes 0x1f 0x8b)
+			// - zlib-compressed protobuf (from the serializer exporter, magic bytes 0x78 0x??)
+			newMetrics, err := seriesFromAny(t, metricsBytes, expectedMetrics)
+			require.NoError(t, err)
+			maps.Copy(metricMap, newMetrics)
 		case <-time.After(60 * time.Second):
-			t.Fatalf("did not receive expected metrics after 1m")
+			t.Fatalf("did not receive expected metrics after 1m, missing: %v", missingMetrics(metricMap, expectedMetrics))
+		}
+	}
+}
+
+// seriesFromAny decodes a metric payload from the mock Datadog server, handling both
+// gzip-compressed JSON (DD trace agent format) and zlib-compressed protobuf (serializer exporter format).
+func seriesFromAny(t *testing.T, metricsBytes []byte, expectedMetrics map[string]struct{}) (map[string]series, error) {
+	if len(metricsBytes) >= 2 && metricsBytes[0] == 0x1f && metricsBytes[1] == 0x8b {
+		// gzip magic bytes: trace agent sends gzip-compressed JSON
+		return seriesFromAPIClient(t, metricsBytes, expectedMetrics)
+	}
+	// zlib magic bytes (0x78 0x??): serializer exporter sends zlib-compressed protobuf
+	return seriesFromSerializer(metricsBytes, expectedMetrics)
+}
+
+// seriesSliceFromAny decodes all series from a metric payload, handling both
+// gzip-compressed JSON (DD trace agent format) and zlib-compressed protobuf (serializer exporter format).
+// Unlike seriesFromAny, it returns a slice so duplicate metric names across different resources are preserved.
+func seriesSliceFromAny(t *testing.T, metricsBytes []byte) ([]series, error) {
+	if len(metricsBytes) >= 2 && metricsBytes[0] == 0x1f && metricsBytes[1] == 0x8b {
+		// gzip magic bytes: trace agent sends gzip-compressed JSON
+		var smap seriesSlice
+		gz := getGzipReader(t, metricsBytes)
+		dec := json.NewDecoder(gz)
+		if err := dec.Decode(&smap); err != nil {
+			return nil, err
+		}
+		return smap.Series, nil
+	}
+	// zlib magic bytes (0x78 0x??): serializer exporter sends zlib-compressed protobuf
+	zr, err := zlib.NewReader(bytes.NewReader(metricsBytes))
+	if err != nil {
+		return nil, err
+	}
+	pl := new(gogen.MetricPayload)
+	b, err := io.ReadAll(zr)
+	if err != nil {
+		return nil, err
+	}
+	if err := pl.Unmarshal(b); err != nil {
+		return nil, err
+	}
+	var result []series
+	for _, s := range pl.GetSeries() {
+		points := make([]point, len(s.GetPoints()))
+		for i, p := range s.GetPoints() {
+			points[i] = point{
+				Timestamp: int(p.GetTimestamp()),
+				Value:     p.GetValue(),
+			}
+		}
+		result = append(result, series{
+			Metric: s.GetMetric(),
+			Points: points,
+			Tags:   s.GetTags(),
+		})
+	}
+	return result, nil
+}
+
+func missingMetrics(got map[string]series, expected map[string]struct{}) []string {
+	var missing []string
+	for k := range expected {
+		if _, ok := got[k]; !ok {
+			missing = append(missing, k)
 		}
 	}
+	return missing
 }
 
 func TestIntegrationLogsHostMetadata(t *testing.T) {

exporter/datadogexporter/integrationtest/integration_test_config.yaml

@@ -26,6 +26,7 @@ exporters:
   datadog:
     api:
       key: "aaa"
+    hostname: "test-host"
     tls:
       insecure_skip_verify: true
     host_metadata:

exporter/datadogexporter/integrationtest/integration_test_host_metrics_config.yaml

@@ -10,6 +10,7 @@ exporters:
   datadog:
     api:
       key: "aaa"
+    hostname: "test-host"
     tls:
       insecure_skip_verify: true
     host_metadata:

exporter/datadogexporter/integrationtest/integration_test_internal_metrics_config.yaml

@@ -9,6 +9,7 @@ exporters:
   datadog:
     api:
       key: "aaa"
+    hostname: "test-host"
     tls:
       insecure_skip_verify: true
     host_metadata:

exporter/datadogexporter/integrationtest/integration_test_logs_config.yaml

@@ -9,6 +9,7 @@ exporters:
   datadog:
     api:
       key: "aaa"
+    hostname: "test-host"
     tls:
       insecure_skip_verify: true
     host_metadata:

exporter/datadogexporter/integrationtest/integration_test_toplevel_config.yaml

@@ -15,6 +15,7 @@ exporters:
   datadog:
     api:
       key: "aaa"
+    hostname: "test-host"
     tls:
       insecure_skip_verify: true
     host_metadata: