Mongo doesnt allow periods in usernames (#11872)

* mongo doesnt allow periods in usernames

* Update mongodb.mdx

Update template in docs

* Move replace to the end

* Adding a test for dot replacement

* Create 11872.txt

plugins/database/mongodb/mongodb_test.go

@@ -82,6 +82,23 @@ func TestNewUser_usernameTemplate(t *testing.T) {
 
 			expectedUsernameRegex: "^v-token-testrolenamewit-[a-zA-Z0-9]{20}-[0-9]{10}$",
 		},
+		"default username template with invalid chars": {
+			usernameTemplate: "",
+
+			newUserReq: dbplugin.NewUserRequest{
+				UsernameConfig: dbplugin.UsernameMetadata{
+					DisplayName: "a.bad.account",
+					RoleName:    "a.bad.role",
+				},
+				Statements: dbplugin.Statements{
+					Commands: []string{mongoAdminRole},
+				},
+				Password:   "98yq3thgnakjsfhjkl",
+				Expiration: time.Now().Add(time.Minute),
+			},
+
+			expectedUsernameRegex: "^v-a-bad-account-a-bad-role-[a-zA-Z0-9]{20}-[0-9]{10}$",
+		},
 		"custom username template": {
 			usernameTemplate: "{{random 2 | uppercase}}_{{unix_time}}_{{.RoleName | uppercase}}_{{.DisplayName | uppercase}}",