Akka http prometheus request latency and count instrumentation.

example.scala

val requestCount = io.prometheus.client.Counter.build().name("http_requests").help("HTTP requests").register()
val requestLatency = io.prometheus.client.Histogram.build().name("http_latency").help("HTTP requests latency").labelNames("code").register()

/*
Example usage:

trackPrometheusMetrics {
  complete("OK")
}
*/
def trackPrometheusMetrics(inner: Route) = {
  // "extract" request start time
  extract(_ => System.currentTimeMillis()){ start =>
    // increment rqt counter
    requestCount.inc()
    // abuse mapResponse to add our side effect
    mapResponse({ r =>
      requestLatency
        .labels(r.status.intValue().toString)
        .observe((System.currentTimeMillis() - start).toDouble)
      r
    }){
      inner
    }
  }
}