[receiver/datadogreceiver] Fix /api/v1/check_run response to return JSON status: ok (#36029)

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

Changing the `/api/v1/check_run` response for service checks to be on
par with the datadog api spec as per
https://docs.datadoghq.com/api/latest/service-checks/

This is the same implementation as the PR for the series endpoints
#35744

<!-- Issue number (e.g. #1234) or full URL to issue, if applicable. -->
#### Link to tracking issue

Fixes
#36027

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

Unit tests updated and are passing

<!--Describe the documentation added.-->
#### Documentation

Changelog updated

Author: jdgeisler

.chloggen/fix-dd-service-checks-response.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/check_run` 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: [36027]
+
+# (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

@@ -361,8 +361,12 @@ func (ddr *datadogReceiver) handleCheckRun(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)
 }
 
 // handleSketches handles sketches, the underlying data structure of distributions https://docs.datadoghq.com/metrics/distributions/

receiver/datadogreceiver/receiver_test.go

@@ -659,7 +659,7 @@ func TestDatadogServices_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()