Is your feature request related to a problem? Please describe.
Currently, metric msg_backlog contains delayed messages and un-delayed messages. However, users need the number of non-delayed messages in the backlog, which can use by too many un-delayed backlogs alerts. So it's better to add msg_backlog_non_delayed metric.

Discussion at slack channel: https://apache-pulsar.slack.com/archives/C5Z4T36F7/p1581283048289800