From 9f9017f26ac612e3134a10c185bc06b8ff5e4fe4 Mon Sep 17 00:00:00 2001
From: Simon Cruanes <simon.cruanes.2007@m4x.org>
Date: Thu, 18 Jan 2024 21:25:49 -0500
Subject: [PATCH] add prometheus middleware for httpd

---
 src/prometheus/tiny_httpd_prometheus.ml  | 34 ++++++++++++++++++------
 src/prometheus/tiny_httpd_prometheus.mli | 18 +++++++------
 tests/prometheus/t_prom.ml               |  8 +++---
 3 files changed, 40 insertions(+), 20 deletions(-)

diff --git a/src/prometheus/tiny_httpd_prometheus.ml b/src/prometheus/tiny_httpd_prometheus.ml
index 026af9db..d9157ddf 100644
--- a/src/prometheus/tiny_httpd_prometheus.ml
+++ b/src/prometheus/tiny_httpd_prometheus.ml
@@ -41,10 +41,10 @@ module Counter = struct
     bpf buf "%s%a %d\n" self.name emit_tags_ self.tags (A.get self.c);
     ()
 
-  let[@inline] inc self = A.incr self.c
-  let[@inline] inc_by self n = ignore (A.fetch_and_add self.c n : int)
-  let[@inline] dec self = A.decr self.c
-  let[@inline] dec_by self n = ignore (A.fetch_and_add self.c (-n) : int)
+  let[@inline] incr self = A.incr self.c
+  let[@inline] incr_by self n = ignore (A.fetch_and_add self.c n : int)
+  let[@inline] decr self = A.decr self.c
+  let[@inline] decr_by self n = ignore (A.fetch_and_add self.c (-n) : int)
 end
 
 module Gauge = struct
@@ -63,10 +63,10 @@ module Gauge = struct
     ()
 
   let[@inline] set self x = A.set self.g x
-  let[@inline] inc self = A.incr self.g
-  let[@inline] inc_by self n = ignore (A.fetch_and_add self.g n : int)
-  let[@inline] dec self = A.decr self.g
-  let[@inline] dec_by self n = ignore (A.fetch_and_add self.g (-n) : int)
+  let[@inline] incr self = A.incr self.g
+  let[@inline] incr_by self n = ignore (A.fetch_and_add self.g n : int)
+  let[@inline] decr self = A.decr self.g
+  let[@inline] decr_by self n = ignore (A.fetch_and_add self.g (-n) : int)
 end
 
 module Registry = struct
@@ -86,3 +86,21 @@ module Registry = struct
 end
 
 let global = Registry.create ()
+
+module H = Tiny_httpd
+
+let http_middleware (reg : Registry.t) : H.Middleware.t =
+  let c_req =
+    Counter.create reg "tiny_httpd_requests" ~descr:"number of HTTP requests"
+  in
+  let c_err =
+    Counter.create reg "tiny_httpd_errors" ~descr:"number of HTTP errors"
+  in
+
+  fun h : H.Middleware.handler ->
+    fun req ~resp : unit ->
+     Counter.incr c_req;
+     h req ~resp:(fun (response : H.Response.t) ->
+         let code = response.code in
+         if code < 200 || code >= 300 then Counter.incr c_err;
+         resp response)
diff --git a/src/prometheus/tiny_httpd_prometheus.mli b/src/prometheus/tiny_httpd_prometheus.mli
index 0cd603f8..b295f687 100644
--- a/src/prometheus/tiny_httpd_prometheus.mli
+++ b/src/prometheus/tiny_httpd_prometheus.mli
@@ -24,10 +24,10 @@ module Counter : sig
   (** A counter, monotonically increasing *)
 
   val create : Registry.t -> ?tags:tags -> ?descr:string -> string -> t
-  val inc : t -> unit
-  val inc_by : t -> int -> unit
-  val dec : t -> unit
-  val dec_by : t -> int -> unit
+  val incr : t -> unit
+  val incr_by : t -> int -> unit
+  val decr : t -> unit
+  val decr_by : t -> int -> unit
 end
 
 (** Gauges *)
@@ -37,13 +37,15 @@ module Gauge : sig
 
   val create : Registry.t -> ?tags:tags -> ?descr:string -> string -> t
   val set : t -> int -> unit
-  val inc : t -> unit
-  val inc_by : t -> int -> unit
-  val dec : t -> unit
-  val dec_by : t -> int -> unit
+  val incr : t -> unit
+  val incr_by : t -> int -> unit
+  val decr : t -> unit
+  val decr_by : t -> int -> unit
 end
 
 (* TODO:
    module Histogram : sig
    end
 *)
+
+val http_middleware : Registry.t -> Tiny_httpd.Middleware.t
diff --git a/tests/prometheus/t_prom.ml b/tests/prometheus/t_prom.ml
index 8e9fe8b5..f8144b7b 100644
--- a/tests/prometheus/t_prom.ml
+++ b/tests/prometheus/t_prom.ml
@@ -8,16 +8,16 @@ let g1 = P.Gauge.create reg ~tags:[ "level", "max" ] "yolo_gauge"
 
 let () =
   print_endline "==== first try ====";
-  P.Counter.inc_by c1 42;
-  P.Counter.inc c2;
+  P.Counter.incr_by c1 42;
+  P.Counter.incr c2;
   P.Gauge.set g1 2525;
 
   pf "```\n%s\n```\n" @@ P.Registry.emit_str reg
 
 let () =
   print_endline "==== second try====";
-  P.Counter.inc_by c1 11;
-  P.Counter.inc c2;
+  P.Counter.incr_by c1 11;
+  P.Counter.incr c2;
   P.Gauge.set g1 42_000;
 
   pf "```\n%s\n```\n" @@ P.Registry.emit_str reg