Support prometheus metrics for udf/udsink containers

[dseapy]

Summary

Users developing udf or udsink applications may want to expose their own prometheus metrics. It would be nice if was an way to do that along with documentation and examples in the SDKs.

Use Cases

Numaflow provides things like how long it took to process datum, but not what parts of my udf are taking a long time. This results in me needing to log this information, which is not ideal. For this use-case I'd like to use my language's prometheus' client library to provide these metrics. Then i'd like to be able to query those metrics similar to how i currently query the numaflow-provided metrics.

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