[MM-28692] Include config diffs in audit record for config changing API calls (#17623)

* Replace config generator

* Cleanup

* Some renaming and docs additions to add clarity

* Cleanup logging related methods

* Cleanup emitter

* Fix TestDefaultsGenerator

* Move feature flags synchronization logic out of config package

* Remove unnecessary util functions

* Simplify load/set logic

* Refine semantics and add some test to cover them

* Remove unnecessary deep copies

* Improve logic further

* Fix license header

* Review file store tests

* Fix test

* Fix test

* Avoid additional write during initialization

* More consistent naming

* Update app/feature_flags.go

Co-authored-by: Christopher Speller <crspeller@gmail.com>

* Update config/store.go

Co-authored-by: Christopher Speller <crspeller@gmail.com>

* Update config/store.go

Co-authored-by: Christopher Speller <crspeller@gmail.com>

* Update config/store.go

Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>

* Make ConfigStore.Set() return both old and new configs

* Implement config diff function

* Make app.SaveConfig return previous and current configs

* Add config diff to audit record

* Fix returned configs

* Include high level test

* Move FF synchronizer to its own package

* Remove unidiomatic use of sync.Once

* Add some comments

* Rename function

* More comment

* Save config diff in audit record for local endpoints

* Enable audit for config set/reset commands

* Improve tests output

Co-authored-by: Christopher Speller <crspeller@gmail.com>
Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>

api4/config.go

@@ -143,13 +143,22 @@ func updateConfig(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	err = c.App.SaveConfig(cfg, true)
+	oldCfg, newCfg, err := c.App.SaveConfig(cfg, true)
 	if err != nil {
 		c.Err = err
 		return
 	}
 
-	cfg, mergeErr := config.Merge(&model.Config{}, c.App.GetSanitizedConfig(), &utils.MergeConfig{
+	diffs, diffErr := config.Diff(oldCfg, newCfg)
+	if diffErr != nil {
+		c.Err = model.NewAppError("updateConfig", "api.config.update_config.diff.app_error", nil, diffErr.Error(), http.StatusInternalServerError)
+		return
+	}
+	auditRec.AddMeta("diff", diffs)
+
+	newCfg.Sanitize()
+
+	cfg, mergeErr := config.Merge(&model.Config{}, newCfg, &utils.MergeConfig{
 		StructFieldFilter: func(structField reflect.StructField, base, patch reflect.Value) bool {
 			return readFilter(c, structField)
 		},
@@ -253,15 +262,24 @@ func patchConfig(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	err = c.App.SaveConfig(updatedCfg, true)
+	oldCfg, newCfg, err := c.App.SaveConfig(updatedCfg, true)
 	if err != nil {
 		c.Err = err
 		return
 	}
 
+	diffs, diffErr := config.Diff(oldCfg, newCfg)
+	if diffErr != nil {
+		c.Err = model.NewAppError("patchConfig", "api.config.patch_config.diff.app_error", nil, diffErr.Error(), http.StatusInternalServerError)
+		return
+	}
+	auditRec.AddMeta("diff", diffs)
+
+	newCfg.Sanitize()
+
 	auditRec.Success()
 
-	cfg, mergeErr = config.Merge(&model.Config{}, c.App.GetSanitizedConfig(), &utils.MergeConfig{
+	cfg, mergeErr = config.Merge(&model.Config{}, newCfg, &utils.MergeConfig{
 		StructFieldFilter: func(structField reflect.StructField, base, patch reflect.Value) bool {
 			return readFilter(c, structField)
 		},