refactor(test): provide means to validate metrics and observations

Some helpers are provided for introspecting metrics already (used in JWT cache tests). This change provides facilities to additionally validate emited Observation events.
A new Spec module is also implemented, adding basic tests of schema cache reloading - their main goal is to excercise the new infrastructure.

src/PostgREST/AppState.hs

@@ -15,6 +15,7 @@ module PostgREST.AppState
   , getJwtCacheState
   , init
   , initWithPool
+  , putConfig -- For tests TODO refactoring
   , putNextListenerDelay
   , putSchemaCache
   , putPgVersion

src/PostgREST/Observation.hs

@@ -1,4 +1,5 @@
-{-# LANGUAGE LambdaCase #-}
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE LambdaCase    #-}
 {-|
 Module      : PostgREST.Observation
 Description : This module holds an Observation type which is the core of Observability for PostgREST.
@@ -65,6 +66,7 @@ data Observation
   | JwtCacheEviction
   | TerminationUnixSignalObs Text
   | WarpErrorObs Text
+  deriving (Generic)
 
 data ObsFatalError = ServerAuthError | ServerPgrstBug | ServerError42P05 | ServerError08P01