Sum /facts aggregates across backends

A `/facts` aggregate row carries no certname, so the per-certname fact
merge collapsed every backend's rows into one bucket and served a single
backend's numbers.

- Route a `/facts` query whose `extract` carries a `function` column to
  serveSummed, as /nodes, /resources and /reports already do
- Document which extract functions combine correctly across backends

server.go

@@ -131,7 +131,7 @@ func (s *Server) handleQuery(w http.ResponseWriter, r *http.Request) {
 	case resourcesPath:
 		s.serveResources(w, r)
 	case factsPath:
-		s.serveMerged(w, r, factsPath, s.mergeFactsResponse(r))
+		s.serveFacts(w, r)
 	case reportsPath:
 		s.serveReports(w, r)
 	case eventsPath:
@@ -212,6 +212,15 @@ func (s *Server) serveUnion(w http.ResponseWriter, r *http.Request, path string,
 	})
 }
 
+// Aggregate rows carry no certname, so the per-certname fact merge would keep only one backend's; they take the summing path instead.
+func (s *Server) serveFacts(w http.ResponseWriter, r *http.Request) {
+	if spec := parseAggregate(r.URL.Query().Get("query")); spec != nil {
+		s.serveSummed(w, r, factsPath, spec.columns)
+		return
+	}
+	s.serveMerged(w, r, factsPath, s.mergeFactsResponse(r))
+}
+
 // A count row carries no certname, so the certname-keyed merge would collapse every backend's count into one backend's; aggregates take the summing path instead.
 func (s *Server) serveNodes(w http.ResponseWriter, r *http.Request) {
 	if spec := parseAggregate(r.URL.Query().Get("query")); spec != nil {