Add built-in Prometheus metrics endpoint

[Denchick]

Currently, spqr-router and spqr-coordinator does not expose any metrics over HTTP. It would be extremely helpful to have a built-in /metrics endpoint exporting router/coordinator/both statistics in Prometheus-compatible format.

Although many useful runtime stats are already available via the SHOW command, they are only accessible via SQL and cannot be easily integrated with standard monitoring systems.

I generated some examples:

var (
	activeClientConnectionsMetric = promauto.NewGauge(prometheus.GaugeOpts{
		Name: "spqr_active_client_connections",
		Help: "The number of active client connections (from SHOW clients).",
	})

	activeBackendConnectionsMetric = promauto.NewGauge(prometheus.GaugeOpts{
		Name: "spqr_active_backend_connections",
		Help: "The number of active backend connections (from SHOW backend_connections).",
	})

	routerUptimeSecondsMetric = promauto.NewGauge(prometheus.GaugeOpts{
		Name: "spqr_router_uptime_seconds",
		Help: "Router uptime in seconds (derived from SHOW status).",
	})

	poolSizeMetric = promauto.NewGaugeVec(prometheus.GaugeOpts{
		Name: "spqr_pool_size",
		Help: "The number of connections in each pool (from SHOW pools).",
	}, []string{"pool_name"})

	queryCountMetric = promauto.NewCounter(prometheus.CounterOpts{
		Name: "spqr_queries_total",
		Help: "Total number of SQL queries processed by the router (derived from SHOW time_quantiles or logs).",
	})

	queryLatencyQuantilesMetric = promauto.NewHistogram(prometheus.HistogramOpts{
		Name:    "spqr_query_latency_seconds",
		Help:    "Query latency distribution (based on SHOW time_quantiles).",
		Buckets: prometheus.DefBuckets,
	})

	moveTasksInProgressMetric = promauto.NewGauge(prometheus.GaugeOpts{
		Name: "spqr_move_tasks_in_progress",
		Help: "Number of ongoing move tasks (from SHOW move_task).",
	})
)