Add telemetry and observability to daemon (bd-153)

Implement comprehensive metrics collection for the daemon with zero-overhead design:

Features:
- Request metrics: counts, latency percentiles (p50, p95, p99), error rates
- Cache metrics: hit/miss ratios, eviction counts, database connections
- Connection metrics: total, active, rejected connections
- System metrics: memory usage, goroutine count, uptime

Implementation:
- New internal/rpc/metrics.go with Metrics collector
- OpMetrics RPC operation for programmatic access
- 'bd daemon --metrics' command (human-readable and JSON output)
- Lock-free atomic operations for cache/connection metrics
- Copy-and-compute pattern in Snapshot to minimize lock contention
- Deferred metrics recording ensures all requests are tracked

Improvements from code review:
- JSON types use float64 for ms/seconds (not time.Duration)
- Snapshot copies data under short lock, computes outside
- Union of operations from counts and errors maps
- Defensive clamping in percentile calculation
- Defer pattern ensures metrics recorded even on early returns

Documentation updated in README.md with usage examples.

Closes bd-153

Amp-Thread-ID: https://ampcode.com/threads/T-20213187-65c7-47f7-ba21-5234c9e52e26
Co-authored-by: Amp <amp@ampcode.com>

internal/rpc/protocol.go

@@ -10,6 +10,7 @@ import (
 const (
 	OpPing           = "ping"
 	OpHealth         = "health"
+	OpMetrics        = "metrics"
 	OpCreate         = "create"
 	OpUpdate         = "update"
 	OpClose          = "close"