Add 'on-request' Hook to Ring Middlewares (#2)

* add 'on-request' hook to 'wrap-metrics' and 'wrap-metrics-expose' middlewares.

* clarify ':on-request' behaviour.

* simplify call to 'on-request' by adding a default binding.

src/iapetos/collector/ring.clj

@@ -142,14 +142,21 @@
 ;; ### Metrics Endpoint
 
 (defn wrap-metrics-expose
-  "Expose Prometheus metrics at the given constant URI using the text format."
+  "Expose Prometheus metrics at the given constant URI using the text format.
+
+   If `:on-request` is given, it will be called with the collector registry
+   whenever a request comes in (the result will be ignored). This lets you use
+   the Prometheus scraper as a trigger for metrics collection."
   [handler registry
-   & [{:keys [path]
-       :or {path "/metrics"}}]]
+   & [{:keys [path on-request]
+       :or {path       "/metrics"
+            on-request identity}}]]
   (fn [{:keys [request-method uri] :as request}]
     (if (= uri path)
       (if (= request-method :get)
-        (metrics-response registry)
+        (do
+          (on-request registry)
+          (metrics-response registry))
         {:status 405})
       (handler request))))
 
@@ -165,7 +172,7 @@
    the `:path` label) if you have any kind of ID in your URIs – since otherwise
    there will be one timeseries created for each observed ID."
   [handler registry
-   & [{:keys [path path-fn]
+   & [{:keys [path path-fn on-request]
        :or {path    "/metrics"
             path-fn :uri}
        :as options}]]

test/iapetos/collector/ring_test.clj

@@ -87,3 +87,20 @@
                   (contains? headers "Content-Type")
                   (re-matches #"text/plain(;.*)?" (headers "Content-Type"))
                   (= (export/text-format registry) body)))))))
+
+(defspec t-wrap-metrics-expose-with-on-request-hook 10
+  (prop/for-all
+    [registry-fn (g/registry-fn ring/initialize)
+     path        (gen/fmap #(str "/" %) gen/string-alpha-numeric)
+     wrap        (gen/elements [ring/wrap-metrics-expose ring/wrap-metrics])]
+    (let [registry (-> (registry-fn)
+                       (prometheus/register
+                         (prometheus/counter :http/requests-total)))
+          on-request-fn #(prometheus/inc % :http/requests-total)
+          handler (-> (constantly {:status 200})
+                      (wrap registry
+                            {:path path
+                             :on-request on-request-fn}))]
+      (and (zero? (prometheus/value (registry :http/requests-total)))
+           (= 200 (:status (handler {:request-method :get, :uri path})))
+           (= 1.0 (prometheus/value (registry :http/requests-total)))))))