feat: add more perf counters to Server-Timing (#2983)

src/PostgREST/Response/Performance.hs

@@ -0,0 +1,34 @@
+module PostgREST.Response.Performance
+  ( ServerMetric(..)
+  , ServerTimingData
+  , renderServerTimingHeader
+  )
+where
+import qualified Data.ByteString.Char8 as BS
+import qualified Data.Map              as Map
+import qualified Network.HTTP.Types    as HTTP
+import           Numeric               (showFFloat)
+import           Protolude
+
+data ServerMetric =
+    SMJwt
+  | SMRender
+  | SMPlan
+  | SMQuery
+  deriving (Show, Eq, Ord)
+type ServerTimingData = Map ServerMetric (Maybe Double)
+
+-- | Render the Server-Timing header from a ServerTimingData
+--
+-- >>> renderServerTimingHeader $ Map.fromList [(SMPlan, 0.1), (SMQuery, 0.2), (SMRender, 0.3), (SMJwt, 0.4)]
+-- ("Server-Timing","jwt;dur=400000.0, render;dur=300000.0, plan;dur=100000.0, query;dur=200000.0")
+renderServerTimingHeader :: ServerTimingData -> HTTP.Header
+renderServerTimingHeader timingData =
+  ("Server-Timing", BS.intercalate ", " $ map renderTiming $ Map.toList timingData)
+renderTiming :: (ServerMetric, Maybe Double) -> BS.ByteString
+renderTiming (metric, time) = maybe "" (\x -> BS.concat [renderMetric metric, BS.pack $ ";dur=" <> showFFloat (Just 1) (x * 1000000) ""]) time
+  where
+    renderMetric SMPlan   = "plan"
+    renderMetric SMQuery  = "query"
+    renderMetric SMRender = "render"
+    renderMetric SMJwt    = "jwt"