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

**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**

```java
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.