merge-stats-map.clj

(def msgs [ {"a" {:stat1 1 :stat2 10} "b" {:stat1 1}} {"a" {:stat1 1}}])

(def merged (apply merge-with (partial merge-with +) msgs))
;; {"a" {:stat1 2, :stat2 10}, "b" {:stat1 1}}
(prn merged)
;; {"a" {:stat1 2, :stat2 10}, "b" {:stat1 1}}

(= (-> merged first second :stat1) 2)
;; true