diff --git a/src/Network/Wai/Middleware/Prometheus.hs b/src/Network/Wai/Middleware/Prometheus.hs
--- a/src/Network/Wai/Middleware/Prometheus.hs
+++ b/src/Network/Wai/Middleware/Prometheus.hs
@@ -2,21 +2,23 @@
 -- metrics and for instrumenting WAI applications.
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE RecordWildCards #-}
-module Network.Wai.Middleware.Prometheus (
-    prometheus
-,   PrometheusSettings (..)
-,   Default.def
-,   instrumentApp
-,   instrumentIO
-,   metricsApp
-) where
 
-import qualified Data.ByteString.Builder as BS
-import qualified Data.ByteString.Char8 as BS
+module Network.Wai.Middleware.Prometheus
+  ( prometheus
+  , PrometheusSettings(..)
+  , Default.def
+  , instrumentHandlerValue
+  , instrumentApp
+  , instrumentIO
+  , metricsApp
+  ) where
+
 import qualified Data.Default as Default
 import Data.Maybe (fromMaybe)
 import Data.Ratio ((%))
+import Data.Text (Text)
 import qualified Data.Text as T
+import Data.Text.Encoding (decodeUtf8)
 import qualified Network.HTTP.Types as HTTP
 import qualified Network.Wai as Wai
 import qualified Prometheus as Prom
@@ -47,28 +49,41 @@
     }
 
 {-# NOINLINE requestLatency #-}
-requestLatency :: Prom.Metric (Prom.Vector Prom.Label3 Prom.Histogram)
-requestLatency = Prom.unsafeRegisterIO $ Prom.vector ("handler", "method", "status_code")
-                                       $ Prom.histogram info Prom.defaultBuckets
+requestLatency :: Prom.Vector Prom.Label3 Prom.Histogram
+requestLatency = Prom.unsafeRegister $ Prom.vector ("handler", "method", "status_code")
+                                     $ Prom.histogram info Prom.defaultBuckets
     where info = Prom.Info "http_request_duration_seconds"
                            "The HTTP request latencies in seconds."
 
+-- | This function is used to populate the @handler@ label of all Prometheus metrics recorded by this library.
+--
+-- If you use this function you will likely want to override the default value
+-- of 'prometheusInstrumentApp' to be false so that your app does not get double
+-- instrumented.
+instrumentHandlerValue ::
+     (Wai.Request -> Text) -- ^ The function used to derive the "handler" value in Prometheus
+  -> Wai.Application -- ^ The app to instrument
+  -> Wai.Application -- ^ The instrumented app
+instrumentHandlerValue f app req respond = do
+  start <- getTime Monotonic
+  app req $ \res -> do
+    end <- getTime Monotonic
+    let method = Just $ decodeUtf8 (Wai.requestMethod req)
+    let status = Just $ T.pack (show (HTTP.statusCode (Wai.responseStatus res)))
+    observeSeconds (f req) method status start end
+    respond res
+
 -- | Instrument a WAI app with the default WAI metrics.
 --
 -- If you use this function you will likely want to override the default value
 -- of 'prometheusInstrumentApp' to be false so that your app does not get double
 -- instrumented.
-instrumentApp :: String           -- ^ The label used to identify this app
-              -> Wai.Application  -- ^ The app to instrument
-              -> Wai.Application  -- ^ The instrumented app
-instrumentApp handler app req respond = do
-    start <- getTime Monotonic
-    app req $ \res -> do
-        end <- getTime Monotonic
-        let method = Just $ BS.unpack (Wai.requestMethod req)
-        let status = Just $ show (HTTP.statusCode (Wai.responseStatus res))
-        observeSeconds handler method status start end
-        respond res
+instrumentApp ::
+     Text -- ^ The label used to identify this app
+  -> Wai.Application -- ^ The app to instrument
+  -> Wai.Application -- ^ The instrumented app
+instrumentApp handler app req respond =
+  instrumentHandlerValue (const handler) app req respond
 
 -- | Instrument an IO action with timing metrics. This function can be used if
 -- you would like to get more fine grained metrics, for instance this can be
@@ -77,7 +92,7 @@
 -- If you use this function you will likely want to override the default value
 -- of 'prometheusInstrumentApp' to be false so that your app does not get double
 -- instrumented.
-instrumentIO :: String  -- ^ The label used to identify this IO operation
+instrumentIO :: Text    -- ^ The label used to identify this IO operation
              -> IO a    -- ^ The IO action to instrument
              -> IO a    -- ^ The instrumented app
 instrumentIO label io = do
@@ -87,12 +102,12 @@
     observeSeconds label Nothing Nothing start end
     return result
 
-observeSeconds :: String -> Maybe String -> Maybe String -> TimeSpec -> TimeSpec -> IO ()
+observeSeconds :: Text -> Maybe Text -> Maybe Text -> TimeSpec -> TimeSpec -> IO ()
 observeSeconds handler method status start end = do
     let latency = fromRational $ toRational (toNanoSecs (end `diffTimeSpec` start) % 1000000000)
-    Prom.withLabel (handler, fromMaybe "" method, fromMaybe "" status)
-                   (Prom.observe latency)
-                   requestLatency
+    Prom.withLabel requestLatency
+                   (handler, fromMaybe "" method, fromMaybe "" status)
+                   (flip Prom.observe latency)
 
 -- | Expose Prometheus metrics and instrument an application with some basic
 -- metrics (e.g. request latency).
@@ -102,8 +117,14 @@
         && Wai.pathInfo req == prometheusEndPoint
         -- XXX: Should probably be "metrics" rather than "prometheus", since
         -- "prometheus" can be confused with actual prometheus.
-    then instrumentApp "prometheus" (const respondWithMetrics) req respond
-    else instrumentApp "app" app req respond
+    then
+      if prometheusInstrumentPrometheus
+        then instrumentApp "prometheus" (const respondWithMetrics) req respond
+        else respondWithMetrics respond
+    else
+      if prometheusInstrumentApp
+        then instrumentApp "app" app req respond
+        else app req respond
 
 
 -- | WAI Application that serves the Prometheus metrics page regardless of
@@ -115,6 +136,6 @@
                    -> IO Wai.ResponseReceived
 respondWithMetrics respond = do
     metrics <- Prom.exportMetricsAsText
-    respond $ Wai.responseBuilder HTTP.status200 headers $ BS.byteString metrics
+    respond $ Wai.responseLBS HTTP.status200 headers metrics
     where
         headers = [(HTTP.hContentType, "text/plain; version=0.0.4")]
diff --git a/wai-middleware-prometheus.cabal b/wai-middleware-prometheus.cabal
--- a/wai-middleware-prometheus.cabal
+++ b/wai-middleware-prometheus.cabal
@@ -1,5 +1,5 @@
 name:                wai-middleware-prometheus
-version:             0.3.0
+version:             1.0.0
 synopsis:
     WAI middlware for exposing http://prometheus.io metrics.
 description:
@@ -29,7 +29,7 @@
     , clock
     , data-default
     , http-types
-    , prometheus-client
+    , prometheus-client  >=1.0.0 && <1.1
     , text               >=0.11
     , wai                >=3.0
   ghc-options: -Wall