Measure RPC method durations and counts (use metrics::record_rpc_request)

**ID:** server.metrics.middleware
**Priority:** P2
**Component:** main + metrics

Wrap router.call to record method durations and counts using existing histogram/counters.

### Acceptance Criteria
- All MCP methods increment counters and observe duration
- Metrics exporter shows rpc metrics
- Simple test validates counters increment

**Estimate:** 1
