Added example

examples/src/main/scala/ObservableExample.scala

@@ -0,0 +1,31 @@
+import cats.effect.{IO, IOApp, Resource}
+import io.opentelemetry.api.GlobalOpenTelemetry
+import org.typelevel.otel4s.metrics.ObservableCounter
+import org.typelevel.otel4s.java.OtelJava
+import java.lang.management.ManagementFactory
+import javax.management.{MBeanServer, ObjectName}
+
+object ObservableExample extends IOApp.Simple {
+
+  val mbeanServer: MBeanServer = ManagementFactory.getPlatformMBeanServer
+  val mbeanName = new ObjectName("cats.effect.metrics:type=CpuStarvation")
+
+  def meterResource: Resource[IO, ObservableCounter] =
+    Resource
+      .eval(IO(GlobalOpenTelemetry.get))
+      .evalMap(OtelJava.forAsync[IO])
+      .evalMap(_.meterProvider.get("observable-example"))
+      .flatMap(
+        _.observableCounter("cats-effect-runtime-cpu-starvation-count")
+          .withDescription("CE runtime starvation count")
+          .createWithCallback(obs =>
+            IO(
+              mbeanServer
+                .getAttribute(mbeanName, "CpuStarvationCount")
+                .asInstanceOf[Long]
+              ).flatMap(c => obs.record(c))
+          )
+      )
+
+  def run: IO[Unit] = meterResource.useForever
+}