bytes_sent / bytes_received: clarify per-protocol semantics and improve Grafana guidance

Customers using Grafana dashboards are confused by `bytes_sent` and `bytes_received` metrics — they expect a single aggregate across all protocols (HTTP, MQTT, WebSocket) but the current behavior reports per-protocol. The docs only say "Bytes sent/received by protocol" without explaining the implications.

## Two-part ask

**1. Documentation update** — make the per-protocol nature explicit and document how to aggregate in Grafana:
```promql
sum by (instance) (harperdb_bytes_sent)
```
Clarify what `protocol` label values to expect and what each represents.

**2. Consider whether to add a total/aggregate metric** — an optional `harperdb_bytes_total{direction="sent"}` style metric that already aggregates could reduce Grafana confusion. Evaluate if worth adding alongside the per-protocol ones.

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🤖 Filed by Claude on behalf of Kris.