Otel4s Metrics example (#4804)

Adds an example for otel4s Metrics integration (added in the PR
#4788)

Author: jctim

examples/src/main/scala/sttp/tapir/examples/observability/Otel4sMetricsExample.scala

@@ -0,0 +1,117 @@
+// {cat=Observability; effects=cats-effect; server=Netty; json=circe}: Otel4s collecting metrics
+
+//> using dep com.softwaremill.sttp.tapir::tapir-core:1.11.44
+//> using dep com.softwaremill.sttp.tapir::tapir-netty-server-cats:1.11.44
+//> using dep com.softwaremill.sttp.tapir::tapir-json-circe:1.11.44
+//> using dep com.softwaremill.sttp.tapir::tapir-otel4s-metrics:1.11.44
+//> using dep "org.typelevel::otel4s-oteljava:0.13.1"
+//> using deps io.opentelemetry:opentelemetry-exporter-otlp:1.54.0
+//> using dep "io.opentelemetry:opentelemetry-sdk-extension-autoconfigure:1.54.0"
+//> using dep ch.qos.logback:logback-classic:1.5.18
+
+package sttp.tapir.examples.observability
+
+import cats.effect
+import cats.effect.std.Dispatcher
+import cats.effect.{IO, IOApp}
+import io.circe.generic.auto.*
+import sttp.tapir.*
+import sttp.tapir.generic.auto.*
+import sttp.tapir.json.circe.jsonBody
+import sttp.tapir.server.netty.cats.{NettyCatsServer, NettyCatsServerOptions}
+import sttp.tapir.server.metrics.otel4s.Otel4sMetrics
+import org.slf4j.{Logger, LoggerFactory}
+import org.typelevel.otel4s.metrics.Meter
+import org.typelevel.otel4s.oteljava.OtelJava
+
+import scala.concurrent.duration.*
+import sttp.tapir.server.ServerEndpoint
+
+import scala.io.StdIn
+
+/** This example application demonstrates how to implement metrics in a Scala application using the <a
+  * href="https://github.com/typelevel/otel4s"> otel4s library </a>. More info about oltel4s you may find <a
+  * href="https://typelevel.org/otel4s/">here</a>
+  *
+  * In order to run otel collector and collect metrics in Prometheus format, you can run it using Docker with the following command:
+  * {{{
+  *   cat > otel-config.yaml << 'EOF'
+  *   receivers:
+  *     otlp:
+  *       protocols:
+  *         grpc:
+  *           endpoint: 0.0.0.0:4317
+  *   exporters:
+  *     prometheus:
+  *       endpoint: 0.0.0.0:8889
+  *   service:
+  *     pipelines:
+  *       metrics:
+  *         receivers: [otlp]
+  *         exporters: [prometheus]
+  *   EOF
+  *
+  *   docker run -p 4317:4317 -p 8889:8889 -v $(pwd)/otel-config.yaml:/etc/otelcol-contrib/config.yaml otel/opentelemetry-collector-contrib:latest
+  * }}}
+  * You can find the collected metrics in prometheus format at http://localhost:8889/metrics
+  */
+object Otel4sMetricsExample extends IOApp.Simple:
+  private val logger: Logger = LoggerFactory.getLogger(this.getClass.getName)
+
+  override def run: IO[Unit] = otel
+    .use { otel4s =>
+      otel4s.meterProvider
+        .get("sttp.tapir.examples.observability")
+        .flatMap { case given Meter[IO] => server() }
+    }
+
+  private def server()(using meter: Meter[IO]): IO[Unit] =
+    Dispatcher
+      .parallel[IO]
+      .map(dispatcher =>
+        NettyCatsServer(
+          NettyCatsServerOptions
+            .customiseInterceptors(dispatcher)
+            .metricsInterceptor(Otel4sMetrics.default(meter).metricsInterceptor())
+            .options
+        )
+      )
+      .use { server =>
+        for {
+          bind <- server
+            .port(8080)
+            .host("localhost")
+            .addEndpoint(personEndpoint)
+            .start()
+          _ <- IO
+            .blocking {
+              logger.info(s"""Server started. Try it with: curl -X POST ${bind.hostName}:${bind.port}/person -d '{"name": "Jacob"}'""")
+              logger.info("Press ENTER key to exit.")
+              StdIn.readLine()
+
+            }
+            .guarantee(bind.stop())
+        } yield ()
+      }
+
+  private def otel: effect.Resource[IO, OtelJava[IO]] = {
+    // Please notice in your, production cases could be better to use env variable instead.
+    // Under following link you may find more details about configuration https://typelevel.org/otel4s/sdk/configuration.html
+    System.setProperty("otel.java.global-autoconfigure.enabled", "true")
+    System.setProperty("otel.service.name", "Otel4s-Metrics-Example")
+    OtelJava.autoConfigured[IO]()
+  }
+
+  case class Person(name: String)
+
+  // Simple endpoint returning 200 or 400 response with string body
+  val personEndpoint: ServerEndpoint[Any, IO] =
+    endpoint.post
+      .in("person")
+      .in(jsonBody[Person])
+      .out(stringBody)
+      .errorOut(stringBody)
+      .serverLogic { p =>
+        IO.sleep(3.seconds) *>
+          IO.pure(Either.cond(p.name == "Jacob", "Welcome", "Unauthorized"))
+      }