report: add cgroup metrics in a separate varlink service (#41489)

Add CpuUsage, MemoryUsage, IOReadBytes, IOReadOperations, and
TasksCurrent in a standalone socket-activated varlink service. These
metrics are gathered from the kernel via cgroup files and PID1's only
role is mapping unit names to cgroup paths — a separate process can
query PID1 once for that mapping and then read the cgroup files
directly, minimizing PID1 involvement.

The new systemd-report-cgroup-metrics service listens at
/run/systemd/report/io.systemd.CGroup and exposes:
  - io.systemd.CGroup.CpuUsage
  - io.systemd.CGroup.IOReadBytes
  - io.systemd.CGroup.IOReadOperations
  - io.systemd.CGroup.MemoryUsage (with type=current/available/peak)
  - io.systemd.CGroup.TasksCurrent

This is spun out of #41078 and based on top of it. Will rebase once
that's merged.

src/basic/cgroup-util.c

@@ -1560,6 +1560,24 @@ fail:
         return r;
 }
 
+int cg_get_keyed_attribute_uint64(const char *path, const char *attribute, const char *key, uint64_t *ret) {
+        _cleanup_free_ char *val = NULL;
+        int r;
+
+        assert(key);
+        assert(ret);
+
+        r = cg_get_keyed_attribute(path, attribute, STRV_MAKE(key), &val);
+        if (r < 0)
+                return r;
+
+        r = safe_atou64(val, ret);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to parse value '%s' of key '%s' in cgroup attribute '%s': %m", val, key, attribute);
+
+        return 0;
+}
+
 int cg_mask_to_string(CGroupMask mask, char **ret) {
         _cleanup_free_ char *s = NULL;
         bool space = false;

src/basic/cgroup-util.h

@@ -165,6 +165,7 @@ int cg_get_attribute_as_uint64(const char *path, const char *attribute, uint64_t
 int cg_get_attribute_as_bool(const char *path, const char *attribute);
 
 int cg_get_keyed_attribute(const char *path, const char *attribute, char * const *keys, char **values);
+int cg_get_keyed_attribute_uint64(const char *path, const char *attribute, const char *key, uint64_t *ret);
 
 int cg_get_owner(const char *path, uid_t *ret_uid);