report: many smaller clean-ups/tweaks to systemd-report (#40598)

/cc @keszybz

diff --cc TODO
index 0e5ef6cd0022eac75713aab41a7311ddcd4bddac,699f514274d0cfbeeaae68839bd595c8d17a0620..e800ded4cbb68486f71797955a67df579fab9b00
--- 1/TODO
--- 2/TODO
+++ b/TODO
@@@ -116,11 -116,16 +116,19 @@@ Deprecations and removals
  
  * Consider removing root=gpt-auto, and push people to use root=dissect instead.
  
 +* remove any trace of "cpuacct" cgroup controller, it's a cgroupv1 thing.
 +  similar "devices"
 +
  Features:
  
+ * report:
+   - should the list of metrics use JSON-SEQ? or maybe be wrapped in a json
+     array (the latter might be necessary, once we sign the combination)
+   - "io.systemd.Manager.unit_active_state" is a weird mix of CamelCase and snake_case
+   - metrics from pid1: suppress metrics form units that are inactive and have nothing to report
+   - how to plug facts into this? i.e. hostname, ssh keys, and so on
+   - switch to daan's suggested hierarchy?
+ 
  * mountfsd: when looking for non-foreign uid owned inodes up the tree, stop at
    mounts