[receiver/datadogreceiver] Fix 202 response for series endpoints to b…

…e on par with DD api spec (#35744)

#### Description
Changes the responses to be on par with the datadog api spec as per:
https://docs.datadoghq.com/api/latest/metrics/?code-lang=go

#### Link to tracking issue
Fixes #35743

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

<!--Describe the documentation added.-->
#### Documentation
Added changelog, no other docs seem to be required

.chloggen/wvanrij-dd-receiver-task_35743.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: 'bug_fix'
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: 'datadogreceiver'
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: "Changes response message for `/api/v1/series` and `/api/v2/series` 202 response to be JSON and on par with Datadog API spec"
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [35743]
+
+# (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/datadogreceiver/receiver.go

@@ -282,8 +282,12 @@ func (ddr *datadogReceiver) handleV1Series(w http.ResponseWriter, req *http.Requ
 		return
 	}
 
+	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(http.StatusAccepted)
-	_, _ = w.Write([]byte("OK"))
+	response := map[string]string{
+		"status": "ok",
+	}
+	_ = json.NewEncoder(w).Encode(response)
 }
 
 // handleV2Series handles the v2 series endpoint https://docs.datadoghq.com/api/latest/metrics/#submit-metrics
@@ -312,8 +316,12 @@ func (ddr *datadogReceiver) handleV2Series(w http.ResponseWriter, req *http.Requ
 		return
 	}
 
+	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(http.StatusAccepted)
-	_, _ = w.Write([]byte("OK"))
+	response := map[string]any{
+		"errors": []string{},
+	}
+	_ = json.NewEncoder(w).Encode(response)
 }
 
 // handleCheckRun handles the service checks endpoint https://docs.datadoghq.com/api/latest/service-checks/

receiver/datadogreceiver/receiver_test.go

@@ -293,7 +293,7 @@ func TestDatadogMetricsV1_EndToEnd(t *testing.T) {
 
 	body, err := io.ReadAll(resp.Body)
 	require.NoError(t, multierr.Combine(err, resp.Body.Close()), "Must not error when reading body")
-	require.Equal(t, "OK", string(body), "Expected response to be 'OK', got %s", string(body))
+	require.JSONEq(t, `{"status": "ok"}`, string(body), "Expected JSON response to be `{\"status\": \"ok\"}`, got %s", string(body))
 	require.Equal(t, http.StatusAccepted, resp.StatusCode)
 
 	mds := sink.AllMetrics()
@@ -371,7 +371,7 @@ func TestDatadogMetricsV2_EndToEnd(t *testing.T) {
 
 	body, err := io.ReadAll(resp.Body)
 	require.NoError(t, multierr.Combine(err, resp.Body.Close()), "Must not error when reading body")
-	require.Equal(t, "OK", string(body), "Expected response to be 'OK', got %s", string(body))
+	require.JSONEq(t, `{"errors": []}`, string(body), "Expected JSON response to be `{\"errors\": []}`, got %s", string(body))
 	require.Equal(t, http.StatusAccepted, resp.StatusCode)
 
 	mds := sink.AllMetrics()