Rename Metrics to Monitors and move it second

"Metrics" promised numbers. The tab holds tracked services, tracked processes
and network interfaces - things being WATCHED and their current state, only
one of which is a measurement. Monitors is what the panels actually are, and
it matches the word the agent's own config already uses for them.

Moved to second, ahead of Software. What is being watched on this box is the
question asked right after "what is it", and well before its application
inventory - which is reference data somebody goes looking for deliberately.

Tab order is now Details, Monitors, Software, Activity.

Contract 0.29.0 keeps its number. It is unreleased, so the group vocabulary it
introduces is still being settled and nothing can yet depend on the old name.
Both the server and the client fall back to 'details' for a group they do not
recognise, so a site that deploys these halves out of step gets a misplaced
card rather than a missing one.

Scope: product

frontend/src/components/PluginAssetPanels.vue

@@ -200,8 +200,9 @@ const panels = ref([])
 // has since been renamed or removed silently disappears. That is what happened
 // to tracked services when it moved out of 'actions' - the backend still said
 // 'actions', this build had no such tab, and the card was simply gone with
-// nothing logged. Fall back the same way the server does rather than drop it.
-const RENDERED_GROUPS = ['details', 'software', 'metrics', 'activity']
+// nothing logged. The same applies to the 'metrics' to 'monitors' rename.
+// Fall back the same way the server does rather than drop it.
+const RENDERED_GROUPS = ['details', 'monitors', 'software', 'activity']
 function groupOf(panel) {
   return RENDERED_GROUPS.includes(panel.group) ? panel.group : 'details'
 }