storage controller: prometheus metrics & dashboard

[jcsp]

Extend metrics, including at least:

• HTTP API status counters + latency histograms for the controller's HTTP API
• Latency/error counters tagged by node_id for outbound calls to pageserver
• Database client metrics: error counter, query latency histogram (wrap with_conn)

Then create a re-usable grafana dashboard for the storage controller that takes the controller container name as a dropdown variable.