[jira] [Created] (KAFKA-17895) Expose KeyValueStore's approximateNumEntries as a metric

["Clément MATHIEU (Jira)" <ji...@apache.org>]

Clément MATHIEU created KAFKA-17895:
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             Summary: Expose KeyValueStore's approximateNumEntries as a metric
                 Key: KAFKA-17895
                 URL: https://issues.apache.org/jira/browse/KAFKA-17895
             Project: Kafka
          Issue Type: Improvement
          Components: streams
    Affects Versions: 3.8.1
            Reporter: Clément MATHIEU


Tracking the evolution of a state store's size is often useful.
 
For example, we often use state store to persist pending work and set alert on maximum size because it means the process is falling behind. 

While KafkaStreams exposes many generic state store related or RocksDB specificif metrics; is does not expose KeyValueStore#approximateNumEntries which is a key information.

Is it an oversight or was it a deliberate choice? 

I would be great if this metrics could be added. I assume that both in-memory & rocksdb implementation of approximateNumEntries are fast enought to be used in metrics. 




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