[MM-18898] Stringify plugin.Log* parameters (#12700)

- Add stringutils with method that "stringify" object slices
  - "stringify" means convert each object to its string representation
- Move plugin.Log* implementations to client_rpc, use stringify method before calling server method
- Exclude Log* methods from generated RPC methods
- No signature change for plugin.Log* API
- Add test in plugin_api_test to use plugin.Log* methods with RPC

app/plugin_api_test.go

@@ -1440,6 +1440,17 @@ func TestPluginAPIGetUnsanitizedConfig(t *testing.T) {
 	}
 }
 
+func TestPluginCallLogAPI(t *testing.T) {
+	th := Setup(t).InitBasic()
+	defer th.TearDown()
+	pluginID := "com.mattermost.sample"
+	path, _ := fileutils.FindDir("mattermost-server/app/plugin_api_test")
+	pluginCode, err := ioutil.ReadFile(filepath.Join(path, "plugin_using_log_api.go"))
+	assert.NoError(t, err)
+	setupPluginApiTest(t, string(pluginCode),
+		`{"id": "com.mattermost.sample", "server": {"executable": "backend.exe"}, "settings_schema": {"settings": []}}`, pluginID, th.App)
+}
+
 func TestPluginAddUserToChannel(t *testing.T) {
 	th := Setup(t).InitBasic()
 	defer th.TearDown()

app/plugin_api_test/plugin_using_log_api.go

@@ -0,0 +1,29 @@
+// Copyright (c) 2019-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package main
+
+import (
+	"github.com/mattermost/mattermost-server/plugin"
+	"github.com/pkg/errors"
+)
+
+type PluginUsingLogAPI struct {
+	plugin.MattermostPlugin
+}
+
+type Foo struct {
+	bar float64
+}
+
+func main() {
+	plugin.ClientMain(&PluginUsingLogAPI{})
+}
+
+func (p *PluginUsingLogAPI) OnActivate() error {
+	p.API.LogDebug("LogDebug", "one", 1, "two", "two", "foo", Foo{bar: 3.1416})
+	p.API.LogInfo("LogInfo", "one", 1, "two", "two", "foo", Foo{bar: 3.1416})
+	p.API.LogWarn("LogWarn", "one", 1, "two", "two", "foo", Foo{bar: 3.1416})
+	p.API.LogError("LogError", "error", errors.WithStack(errors.New("boom!")))
+	return nil
+}