A simple and modern, type safe, performance focused, idiomatic Haskell client for Prometheus monitoring. Specifically there is no use of unsafe IO or manual ByteString construction from lists of bytes. Batteries-included web server. A key design element of this library is that the RegistryT monad transformer is only required for registering new time series. Once the time series is registered, new data samples may just be added in the IO monad. Note: Version 0.* supports Prometheus v1.0 and version 2.* supports Prometheus v2.0. A Registry and StateT-based RegistryT are available for unit testing or generating lists of `[IO a]` actions that can be sequenced and returned from pure code to be applied.