Prometheus metric on publisher message compression ratio

Is your feature request related to a problem? Please describe.
In our organisation we sink Pulsar messages for some topics directly to S3. It is useful for us to know the compression ratio that we are achieving on our payload size so that we are maximising efficiency of data storage, as we are able to grow/shrink the payload size to optimise for this.

Describe the solution you'd like
A prometheus histogram or gauge metric that producers publish on each message compression.

Describe alternatives you've considered
We considered calculating this metric ourselves, however the compression code is in the internal directory in the project. I do not think there are any public methods that would give us this data.

Additional context
N/A

The issue names no files, tests, or entry points; begin by locating the publisher's internal message-compression path and existing Prometheus metric definitions. Confirm how compressed and original payload sizes are available, then verify that a per-message compression-ratio histogram or gauge is exposed and covered by appropriate tests.

Written by the indexing model from the issue text.