Hi all, I've started a PIP to discuss: PIP-248 Add backlog eviction metric
### Motivation: Pulsar has 2 configurations for the backlog eviction: `backlogQuotaDefaultLimitBytes` and `backlogQuotaDefaultLimitSecond`, if topic backlog reaches the threshold of any item, backlog eviction will be triggered. Before backlog eviction happens, we don't have a metric to monitor how long that it can reaches the threshold. We can provide a progress bar metric to tell users some topics is about to trigger backlog eviction. And users can subscribe the alert to schedule consumers. For more details, please read the PIP at https://github.com/apache/pulsar/issues/19601 Thanks, Tao Jiuming
