[Vote] PIP-248: Add metrics for backlog eviction events

[太上玄元道君]

Hi all,

The thread is to start the vote for PIP-248

Discussion thread:
https://lists.apache.org/thread/m7khb49pk7vdx5yxvhzhkbo7t48d0rzs
PIP link: https://github.com/apache/pulsar/issues/19601

Thanks,
Tao Jiuming