Add per-sandbox CPU/memory/disk metrics collection

Samples /proc/{fc_pid}/stat (CPU%), /proc/{fc_pid}/status (VmRSS), and
stat() on CoW files at 500ms intervals per running sandbox. Three tiered
ring buffers downsample into 30s and 5min averages for 10min/2h/24h
retention. Metrics are flushed to DB on pause (all tiers) and destroy
(24h only). New GetSandboxMetrics and FlushSandboxMetrics RPCs on the
host agent, proxied through GET /v1/sandboxes/{id}/metrics?range= on
the control plane. Returns live data for running sandboxes, DB data for
paused, and 404 for stopped.

internal/sandbox/proc.go

@@ -0,0 +1,105 @@
+package sandbox
+
+import (
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+	"syscall"
+)
+
+// findChildPID reads the direct child PID of a given parent process.
+// The Firecracker process is a direct child of the unshare wrapper because
+// the init script uses `exec ip netns exec ... firecracker`, which replaces
+// bash with ip-netns-exec, which in turn execs firecracker — same PID,
+// direct child of unshare.
+func findChildPID(parentPID int) (int, error) {
+	path := fmt.Sprintf("/proc/%d/task/%d/children", parentPID, parentPID)
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return 0, fmt.Errorf("read children: %w", err)
+	}
+	fields := strings.Fields(string(data))
+	if len(fields) == 0 {
+		return 0, fmt.Errorf("no child processes found for PID %d", parentPID)
+	}
+	pid, err := strconv.Atoi(fields[0])
+	if err != nil {
+		return 0, fmt.Errorf("parse child PID %q: %w", fields[0], err)
+	}
+	return pid, nil
+}
+
+// cpuStat holds raw CPU jiffies read from /proc/{pid}/stat.
+type cpuStat struct {
+	utime uint64
+	stime uint64
+}
+
+// readCPUStat reads user and system CPU jiffies from /proc/{pid}/stat.
+// Fields 14 (utime) and 15 (stime) are 1-indexed in the man page;
+// after splitting on space, they are at indices 13 and 14.
+func readCPUStat(pid int) (cpuStat, error) {
+	path := fmt.Sprintf("/proc/%d/stat", pid)
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return cpuStat{}, fmt.Errorf("read stat: %w", err)
+	}
+
+	// /proc/{pid}/stat format: pid (comm) state fields...
+	// The comm field may contain spaces and parens, so find the last ')' first.
+	content := string(data)
+	idx := strings.LastIndex(content, ")")
+	if idx < 0 {
+		return cpuStat{}, fmt.Errorf("malformed /proc/%d/stat: no closing paren", pid)
+	}
+	// After ")" there is " state field3 field4 ... fieldN"
+	// field1 after ')' is state (index 0), utime is field 11, stime is field 12
+	// (0-indexed from after the closing paren).
+	fields := strings.Fields(content[idx+2:])
+	if len(fields) < 13 {
+		return cpuStat{}, fmt.Errorf("malformed /proc/%d/stat: too few fields (%d)", pid, len(fields))
+	}
+	utime, err := strconv.ParseUint(fields[11], 10, 64)
+	if err != nil {
+		return cpuStat{}, fmt.Errorf("parse utime: %w", err)
+	}
+	stime, err := strconv.ParseUint(fields[12], 10, 64)
+	if err != nil {
+		return cpuStat{}, fmt.Errorf("parse stime: %w", err)
+	}
+	return cpuStat{utime: utime, stime: stime}, nil
+}
+
+// readMemRSS reads VmRSS from /proc/{pid}/status and returns bytes.
+func readMemRSS(pid int) (int64, error) {
+	path := fmt.Sprintf("/proc/%d/status", pid)
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return 0, fmt.Errorf("read status: %w", err)
+	}
+	for _, line := range strings.Split(string(data), "\n") {
+		if strings.HasPrefix(line, "VmRSS:") {
+			fields := strings.Fields(line)
+			if len(fields) < 2 {
+				return 0, fmt.Errorf("malformed VmRSS line")
+			}
+			kb, err := strconv.ParseInt(fields[1], 10, 64)
+			if err != nil {
+				return 0, fmt.Errorf("parse VmRSS: %w", err)
+			}
+			return kb * 1024, nil
+		}
+	}
+	return 0, fmt.Errorf("VmRSS not found in /proc/%d/status", pid)
+}
+
+// readDiskAllocated returns the actual allocated bytes (not apparent size)
+// of the file at path. This uses stat's block count × 512.
+func readDiskAllocated(path string) (int64, error) {
+	var stat syscall.Stat_t
+	if err := syscall.Stat(path, &stat); err != nil {
+		return 0, fmt.Errorf("stat %s: %w", path, err)
+	}
+	return stat.Blocks * 512, nil
+}