1503 prometheus example (#1541)

* added prometheus exporter example

* tidying up

* Update examples/prometheus/README.md

Co-authored-by: Giovanni Liva <giovanni.liva@dynatrace.com>

* Update examples/prometheus/README.md

Co-authored-by: Giovanni Liva <giovanni.liva@dynatrace.com>

* added port arg check as per code review suggestion

Co-authored-by: Giovanni Liva <giovanni.liva@dynatrace.com>

examples/prometheus/README.md

@@ -0,0 +1,41 @@
+# Prometheus Example
+
+This example demonstrates how to use the OpenTelemetry SDK 
+to instrument a simple application using Prometheus as the metric exporter and expose the metrics via HTTP. 
+
+These are collected by a Prometheus instance which is configured to pull these metrics via HTTP. 
+
+# How to run
+
+## Prerequisites
+* Java 1.7
+* Docker 19.03
+
+## 1 - Compile 
+```shell script
+../gradlew fatJar
+```
+## 2 - Run Prometheus
+
+Start Prometheus instance with a configuration that sets up a HTTP collection job for  ```127.0.0.1:19090```
+
+See [prometheus.yml](prometheus.yml)
+
+```shell script
+docker run --network="host" --rm -it \
+    --name prometheus \
+    -v $(pwd)/prometheus.yml:/etc/prometheus/prometheus.yml \
+    prom/prometheus 
+
+```
+
+## 3 - Start the Application
+```shell script
+java -cp build/libs/opentelemetry-examples-prometheus-all-0.1.0-SNAPSHOT.jar io.opentelemetry.example.PrometheusExample 19090
+```
+## 4 - Open the Prometheus UI
+
+Navigate to:
+
+http://localhost:9090/graph?g0.range_input=15m&g0.expr=incoming_messages&g0.tab=0
+

examples/prometheus/build.gradle

@@ -0,0 +1,14 @@
+plugins {
+    id 'java'
+}
+
+description = 'OpenTelemetry Example for Prometheus Exporter'
+ext.moduleName = "io.opentelemetry.examples.prometheus"
+
+dependencies {
+    compile("io.opentelemetry:opentelemetry-api:${opentelemetryVersion}")
+    compile("io.opentelemetry:opentelemetry-sdk:${opentelemetryVersion}")
+    compile("io.opentelemetry:opentelemetry-exporters-prometheus:${opentelemetryVersion}")
+    compile("io.prometheus:simpleclient:0.8.1")
+    compile("io.prometheus:simpleclient_httpserver:0.8.1")
+}

examples/prometheus/prometheus.yml

@@ -0,0 +1,30 @@
+global:
+  scrape_interval: 15s
+  scrape_timeout: 10s
+  evaluation_interval: 15s
+alerting:
+  alertmanagers:
+  - static_configs:
+    - targets: []
+    scheme: http
+    timeout: 10s
+    api_version: v1
+scrape_configs:
+- job_name: prometheus
+  honor_timestamps: true
+  scrape_interval: 15s
+  scrape_timeout: 10s
+  metrics_path: /metrics
+  scheme: http
+  static_configs:
+  - targets:
+    - localhost:9090
+- job_name: otel_java_prometheus_example
+  honor_timestamps: true
+  scrape_interval: 15s
+  scrape_timeout: 10s
+  metrics_path: /metrics
+  scheme: http
+  static_configs:
+  - targets:
+    - 127.0.0.1:19090

examples/prometheus/src/main/java/io/opentelemetry/example/PrometheusExample.java

@@ -0,0 +1,86 @@
+package io.opentelemetry.example;
+
+import io.opentelemetry.common.Labels;
+import io.opentelemetry.exporters.prometheus.PrometheusCollector;
+import io.opentelemetry.metrics.AsynchronousInstrument;
+import io.opentelemetry.metrics.LongValueObserver;
+import io.opentelemetry.metrics.Meter;
+import io.opentelemetry.sdk.OpenTelemetrySdk;
+import io.opentelemetry.sdk.metrics.MeterSdkProvider;
+import io.prometheus.client.exporter.HTTPServer;
+import java.io.IOException;
+import java.util.concurrent.ThreadLocalRandom;
+
+/**
+ * Example of using the {@link PrometheusCollector} to convert OTel metrics to Prometheus format and
+ * expose these to a Prometheus instance via a {@link HTTPServer} exporter.
+ *
+ * <p>A {@link LongValueObserver} is used to periodically measure how many incoming messages are
+ * awaiting processing. The {@link LongValueObserver.Callback} gets executed every collection
+ * interval.
+ */
+public class PrometheusExample {
+
+  private final MeterSdkProvider meterSdkProvider = OpenTelemetrySdk.getMeterProvider();
+  private final Meter meter = meterSdkProvider.get("PrometheusExample", "0.7");
+  private final HTTPServer server;
+  private long incomingMessageCount;
+
+  public PrometheusExample(int port) throws IOException {
+
+    LongValueObserver observer =
+        meter
+            .longValueObserverBuilder("incoming.messages")
+            .setDescription("No of incoming messages awaiting processing")
+            .setUnit("message")
+            .build();
+
+    observer.setCallback(
+        new LongValueObserver.Callback<LongValueObserver.LongResult>() {
+          @Override
+          public void update(AsynchronousInstrument.LongResult result) {
+            result.observe(incomingMessageCount, Labels.empty());
+          }
+        });
+
+    PrometheusCollector.newBuilder()
+        .setMetricProducer(meterSdkProvider.getMetricProducer())
+        .buildAndRegister();
+
+    server = new HTTPServer(port);
+  }
+
+  void shutdown() {
+    server.stop();
+  }
+
+  void simulate() {
+    for (int i = 300; i > 0; i--) {
+      try {
+        System.out.println(
+            i + " Iterations to go, current incomingMessageCount is:  " + incomingMessageCount);
+        incomingMessageCount = ThreadLocalRandom.current().nextLong(100);
+        Thread.sleep(1000);
+      } catch (InterruptedException e) {
+      }
+    }
+  }
+
+  public static void main(String[] args) throws IOException {
+
+    int port = 0;
+    try {
+      port = Integer.parseInt(args[0]);
+    } catch (Exception e) {
+      System.out.println("Port not set, or is invalid. Exiting");
+      System.exit(1);
+    }
+
+    PrometheusExample prometheusExample = new PrometheusExample(port);
+
+    prometheusExample.simulate();
+    prometheusExample.shutdown();
+
+    System.out.println("Exiting");
+  }
+}

examples/settings.gradle

@@ -16,6 +16,7 @@ include ":opentelemetry-examples-grpc",
         ":opentelemetry-examples-http",
         ":opentelemetry-examples-jaeger",
         ":opentelemetry-examples-metrics",
+        ":opentelemetry-examples-prometheus",
         ":opentelemetry-examples-sdk-usage",
         ":opentelemetry-examples-zipkin"