Experimental: Add netty backend

Previously we supported blaze (default) and ember as http4s backends.
This adds a new - pure Netty backend. Netty is a non-blocking I/O
client-server framework for the network applications.
There are some significant performance and feature differences.
It provides a good control over request handling and underlying low-level
protocol.

build.sbt

@@ -21,6 +21,7 @@ lazy val core = project
       Dependencies.Libraries.http4sDsl,
       Dependencies.Libraries.http4sBlaze,
       Dependencies.Libraries.http4sEmber,
+      Dependencies.Libraries.http4sNetty,
       Dependencies.Libraries.http4sClient,
       Dependencies.Libraries.log4cats,
       Dependencies.Libraries.thrift,

core/src/main/scala/com.snowplowanalytics.snowplow.collector.core/Config.scala

@@ -182,6 +182,7 @@ object Config {
     object Backend {
       case object Blaze extends Backend
       case object Ember extends Backend
+      case object Netty extends Backend
     }
   }
 
@@ -224,6 +225,7 @@ object Config {
     implicit val backend: Decoder[Experimental.Backend] = Decoder[String].emap {
       case s if s.toLowerCase() == "blaze" => Right(Experimental.Backend.Blaze)
       case s if s.toLowerCase() == "ember" => Right(Experimental.Backend.Ember)
+      case s if s.toLowerCase() == "netty" => Right(Experimental.Backend.Netty)
       case other                           => Left(s"Invalid backend $other")
     }
     implicit val experimental = deriveDecoder[Experimental]

core/src/main/scala/com.snowplowanalytics.snowplow.collector.core/HttpServer.scala

@@ -17,7 +17,8 @@ import com.avast.datadog4s.extension.http4s.DatadogMetricsOps
 import com.avast.datadog4s.{StatsDMetricFactory, StatsDMetricFactoryConfig}
 import org.http4s.{HttpApp, HttpRoutes}
 import org.http4s.blaze.server.BlazeServerBuilder
-import org.http4s.ember.server._
+import org.http4s.ember.server.EmberServerBuilder
+import org.http4s.netty.server.NettyServerBuilder
 import com.comcast.ip4s._
 import fs2.io.net.Network
 import fs2.io.net.tls.TLSContext
@@ -144,6 +145,27 @@ object HttpServer {
           .build
     }
 
+  private def buildNettyServer[F[_]: Async: Network](
+    routes: HttpRoutes[F],
+    port: Int,
+    secure: Boolean,
+    hsts: Config.HSTS,
+    networking: Config.Networking,
+    debugHttp: Config.Debug.Http
+  ): Resource[F, Server] =
+    Resource.eval(TLSContext.Builder.forAsync[F].system).flatMap { tls =>
+      Resource.eval(Logger[F].info("Building netty server")) >>
+        NettyServerBuilder[F]
+          .bindHttp(port)
+          .withHttpApp(
+            loggerMiddleware(timeoutMiddleware(hstsMiddleware(hsts, routes.orNotFound), networking), debugHttp)
+          )
+        .withIdleTimeout(networking.idleTimeout)
+        .withMaxInitialLineLength(networking.maxRequestLineLength)
+        .cond(secure, _.withSslContext(SSLContext.getDefault))
+        .build
+    }
+
   implicit class ConditionalAction[A](item: A) {
     def cond(cond: Boolean, action: A => A): A =
       if (cond) action(item) else item

project/Dependencies.scala

@@ -24,6 +24,7 @@ object Dependencies {
     val decline          = "2.4.1"
     val fs2PubSub        = "0.22.0"
     val http4s           = "0.23.23"
+    val http4sNetty      = "0.5.16"
     val jackson          = "2.12.7" // force this version to mitigate security vulnerabilities
     val fs2Kafka         = "2.6.1"
     val log4cats         = "2.6.0"
@@ -54,6 +55,7 @@ object Dependencies {
     val collectorPayload  = "com.snowplowanalytics" % "collector-payload-1"                    % V.collectorPayload
     val decline           = "com.monovore"          %% "decline-effect"                        % V.decline
     val emitterHttps      = "com.snowplowanalytics" %% "snowplow-scala-tracker-emitter-http4s" % V.tracker
+    val http4sNetty       = "org.http4s"            %% "http4s-netty-server"                   % V.http4sNetty
     val http4sEmber       = "org.http4s"            %% "http4s-ember-server"                   % V.http4s
     val http4sBlaze       = "org.http4s"            %% "http4s-blaze-server"                   % V.blaze
     val http4sClient      = "org.http4s"            %% "http4s-blaze-client"                   % V.blaze