[MM-55268] Implement ServeMetrics plugins hook (#24249)

* Implement ServeMetrics plugins hook

* Update error id

* Simplify

* Revert "Simplify"

This reverts commit c9dc5d5eac6c933ff69e158cf3e34d0973bd3bcd.

* Add comment and error handler

* Wrap error

* Update translation file

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/app/plugin_api_test.go

@@ -2305,3 +2305,63 @@ func TestSendPushNotification(t *testing.T) {
 	}
 	assert.Equal(t, 6, numMessages)
 }
+
+func TestPluginServeMetrics(t *testing.T) {
+	th := Setup(t, StartMetrics)
+	defer th.TearDown()
+
+	var prevEnable *bool
+	var prevAddress *string
+	th.App.UpdateConfig(func(cfg *model.Config) {
+		prevEnable = cfg.MetricsSettings.Enable
+		prevAddress = cfg.MetricsSettings.ListenAddress
+		cfg.MetricsSettings.Enable = model.NewBool(true)
+		cfg.MetricsSettings.ListenAddress = model.NewString(":30067")
+	})
+	defer th.App.UpdateConfig(func(cfg *model.Config) {
+		cfg.MetricsSettings.Enable = prevEnable
+		cfg.MetricsSettings.ListenAddress = prevAddress
+	})
+
+	testFolder, found := fileutils.FindDir("channels/app/plugin_api_tests")
+	require.True(t, found, "Cannot find tests folder")
+	fullPath := path.Join(testFolder, "manual.test_serve_metrics_plugin", "main.go")
+
+	pluginCode, err := os.ReadFile(fullPath)
+	require.NoError(t, err)
+	require.NotEmpty(t, pluginCode)
+
+	tearDown, ids, errors := SetAppEnvironmentWithPlugins(t, []string{string(pluginCode)}, th.App, th.NewPluginAPI)
+	defer tearDown()
+	require.NoError(t, errors[0])
+	require.Len(t, ids, 1)
+
+	pluginID := ids[0]
+	require.NotEmpty(t, pluginID)
+
+	reqURL := fmt.Sprintf("http://localhost%s/plugins/%s/metrics", *th.App.Config().MetricsSettings.ListenAddress, pluginID)
+	req, err := http.NewRequest("GET", reqURL, nil)
+	require.NoError(t, err)
+
+	client := &http.Client{}
+	resp, err := client.Do(req)
+	require.NoError(t, err)
+
+	defer resp.Body.Close()
+
+	body, err := io.ReadAll(resp.Body)
+	require.NoError(t, err)
+	require.Equal(t, "METRICS", string(body))
+
+	reqURL = fmt.Sprintf("http://localhost%s/plugins/%s/metrics/subpath", *th.App.Config().MetricsSettings.ListenAddress, pluginID)
+	req, err = http.NewRequest("GET", reqURL, nil)
+	require.NoError(t, err)
+
+	resp, err = client.Do(req)
+	require.NoError(t, err)
+	defer resp.Body.Close()
+
+	body, err = io.ReadAll(resp.Body)
+	require.NoError(t, err)
+	require.Equal(t, "METRICS SUBPATH", string(body))
+}