diff --git a/app/prometheus_app.ml b/app/prometheus_app.ml
index 3e3382e..5a77914 100644
--- a/app/prometheus_app.ml
+++ b/app/prometheus_app.ml
@@ -46,11 +46,11 @@ module TextFormat_0_0_4 = struct
     | [] -> ()
     | label_values -> Fmt.pf f "{%a}" output_pairs (label_names, label_values)
 
-  let output_sample ~base ~label_names ~label_values f (ext, sample) =
+  let output_sample ~base ~label_names ~label_values f { Sample_set.ext; value } =
     Fmt.pf f "%a%s%a %a@."
       MetricName.pp base ext
       (output_labels ~label_names) label_values
-      output_value sample
+      output_value value
 
   let output_metric ~name ~label_names f (label_values, samples) =
     List.iter (output_sample ~base:name ~label_names ~label_values f) samples
@@ -83,7 +83,7 @@ module Runtime = struct
     }
     in
     let collect () =
-      LabelSetMap.singleton [] ["", fn ()]
+      LabelSetMap.singleton [] [Sample_set.sample (fn ())]
     in
     info, collect
 
diff --git a/app/prometheus_unix.ml b/app/prometheus_unix.ml
index 51217f3..66e852e 100644
--- a/app/prometheus_unix.ml
+++ b/app/prometheus_unix.ml
@@ -13,7 +13,7 @@ module Unix_runtime = struct
     }
     in
     let collect () =
-      LabelSetMap.singleton [] ["", fn ()]
+      LabelSetMap.singleton [] [Sample_set.sample (fn ())]
     in
     info, collect
 
diff --git a/src/prometheus.ml b/src/prometheus.ml
index 8f6c948..d67c8f2 100644
--- a/src/prometheus.ml
+++ b/src/prometheus.ml
@@ -79,13 +79,24 @@ end
 
 module MetricFamilyMap = Map.Make(MetricInfo)
 
+module Sample_set = struct
+  type sample = {
+    ext : string;
+    value : float;
+  }
+
+  type t = sample list
+
+  let sample ?(ext="") value = { ext; value }
+end
+
 module CollectorRegistry = struct
   type t = {
-    mutable metrics : (unit -> (string * float) list LabelSetMap.t) MetricFamilyMap.t;
+    mutable metrics : (unit -> Sample_set.t LabelSetMap.t) MetricFamilyMap.t;
     mutable pre_collect : (unit -> unit) list;
   }
 
-  type snapshot = (string * float) list LabelSetMap.t MetricFamilyMap.t
+  type snapshot = Sample_set.t LabelSetMap.t MetricFamilyMap.t
 
   let create () = {
     metrics = MetricFamilyMap.empty;
@@ -118,7 +129,7 @@ end
 module type CHILD = sig
   type t
   val create : unit -> t
-  val values : t -> (string * float) list       (* extension, value *)
+  val values : t -> Sample_set.t
   val metric_type : metric_type
 end
 
@@ -167,7 +178,7 @@ module Counter = struct
   include Metric(struct
       type t = float ref
       let create () = ref 0.0
-      let values t = ["", !t]
+      let values t = [Sample_set.sample !t]
       let metric_type = Counter
     end)
 
@@ -183,7 +194,7 @@ module Gauge = struct
   include Metric(struct
       type t = float ref
       let create () = ref 0.0
-      let values t = ["", !t]
+      let values t = [Sample_set.sample !t]
       let metric_type = Gauge
     end)
 
@@ -220,8 +231,8 @@ module Summary = struct
     let create () = { count = 0.0; sum = 0.0 }
     let values t =
       [
-        "_sum", t.sum;
-        "_count", t.count;
+        Sample_set.sample ~ext:"_sum" t.sum;
+        Sample_set.sample ~ext:"_count" t.count;
       ]
     let metric_type = Summary
   end
diff --git a/src/prometheus.mli b/src/prometheus.mli
index ab87430..006c3a8 100644
--- a/src/prometheus.mli
+++ b/src/prometheus.mli
@@ -51,11 +51,27 @@ module LabelSetMap : Map.S with type key = string list
 module MetricFamilyMap : Map.S with type key = MetricInfo.t
 (** A map indexed by metric families. *)
 
+module Sample_set : sig
+  type sample = {
+    ext : string;               (** An extension to append to the base metric name. *)
+    value : float;
+  }
+
+  type t = sample list
+  (** A collection of values that together represent a single sample.
+      For a counter, each reading is just a single value, but more complex types
+      require multiple values.
+      For example, a "summary" sample set contains "_sum" and "_count" values.
+   *)
+
+  val sample : ?ext:string -> float -> sample
+end
+
 module CollectorRegistry : sig
   type t
   (** A collection of metrics to be monitored. *)
 
-  type snapshot = (string * float) list LabelSetMap.t MetricFamilyMap.t
+  type snapshot = Sample_set.t LabelSetMap.t MetricFamilyMap.t
   (** The result of reading a set of metrics. *)
 
   val create : unit -> t
@@ -67,7 +83,7 @@ module CollectorRegistry : sig
   val collect : t -> snapshot
   (** Read the current value of each metric. *)
 
-  val register : t -> MetricInfo.t -> (unit -> (string * float) list LabelSetMap.t) -> unit
+  val register : t -> MetricInfo.t -> (unit -> Sample_set.t LabelSetMap.t) -> unit
   (** [register t metric collector] adds [metric] to the set of metrics being collected.
       It will call [collector ()] to collect the values each time [collect] is called. *)