chore: move executable code to src/

src/ now contains all source code - in subdirectories, according to the
.cabal component they belong to. This will allow us to put vendored
libraries in the same place - and later split our own code into multiple
components/libraries as well.

src/library/PostgREST/Response/Performance.hs

@@ -0,0 +1,41 @@
+module PostgREST.Response.Performance
+  ( ServerTiming (..)
+  , serverTimingHeader
+  )
+where
+import qualified Data.ByteString.Char8 as BS
+import qualified Network.HTTP.Types    as HTTP
+import           Numeric               (showFFloat)
+import           Protolude
+
+-- $setup
+-- >>> import Protolude
+
+-- | ServerTiming represents the timing data for a request, in seconds.
+data ServerTiming =
+  ServerTiming
+    { jwt         :: Maybe Double
+    , parse       :: Maybe Double
+    , plan        :: Maybe Double
+    , transaction :: Maybe Double
+    , response    :: Maybe Double
+    }
+  deriving (Show)
+
+-- | Render the Server-Timing header from a ServerTimingData
+-- The duration precision is milliseconds, per the docs
+--
+-- >>> serverTimingHeader ServerTiming { plan=Just 0.1, transaction=Just 0.2, response=Just 0.3, jwt=Just 0.4, parse=Just 0.5}
+-- ("Server-Timing","jwt;dur=0.4, parse;dur=0.5, plan;dur=0.1, transaction;dur=0.2, response;dur=0.3")
+serverTimingHeader :: ServerTiming -> HTTP.Header
+serverTimingHeader timing =
+  ("Server-Timing", renderTiming)
+  where
+    renderMetric metric = maybe "" (\dur -> BS.concat [metric, BS.pack $ ";dur=" <> showFFloat (Just 1) dur ""])
+    renderTiming = BS.intercalate ", " $ (\(k, v) -> renderMetric k (v timing)) <$>
+      [ ("jwt", jwt)
+      , ("parse", parse)
+      , ("plan", plan)
+      , ("transaction", transaction)
+      , ("response", response)
+      ]