Add legacy toggle to clients

lib/gitlab/prometheus_client.rb

@@ -163,7 +163,7 @@ module Gitlab
     end
 
     def parse_json(response_body)
-      Gitlab::Json.parse(response_body)
+      Gitlab::Json.parse(response_body, legacy_mode: true)
     rescue JSON::ParserError
       raise PrometheusClient::Error, 'Parsing response failed'
     end

lib/mattermost/client.rb

@@ -49,7 +49,7 @@ module Mattermost
     end
 
     def json_response(response)
-      json_response = Gitlab::Json.parse(response.body)
+      json_response = Gitlab::Json.parse(response.body, legacy_mode: true)
 
       unless response.success?
         raise Mattermost::ClientError.new(json_response['message'] || 'Undefined error')