[DISCUSS] PIP-399: Fix Metric Name for Delayed Queue

[thetumbled]

Hi, Pulsar Community.
    I open a new PIP to fix metric name for delayed queue, which involves 
changing metirc name and a pip is needed.
    pip link: https://github.com/apache/pulsar/pull/23789
    implementation link: https://github.com/apache/pulsar/pull/23712

Thanks,
Wenzhi Feng(thetumbled)