Task: Add more statistical metrics to the KV storage engine.

[hageshiame]

Is your feature request related to a problem? Please describe.
A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]

Describe the solution you'd like

bifromq-retain/bifromq-retain-client/src/main/java/org/apache/bifromq/retain/client/RetainClient.java


        if (!payload.isEmpty()) {
            tenantMeter.recordSummary(MqttIngressRetainBytes, payload.size());
        }
        // TODO: add throttler
        return rpcClient.invoke(publisher.getTenantId(), null, RetainRequest.newBuilder()
                .setReqId(reqId)
                .setTopic(topic)

• In-memory engine: Requires extended statistics collection (InMemKVSpace.java:101-102)
• RocksDB engine: Also requires more statistical metrics (RocksDBKVSpace.java:186-187)

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.