Prometheus federation example (open-telemetry#57)

* Add an example showing how to use the OpenTelemetry collector to collect metrics from a Prometheus server

* Fix imports

* Remove mention of corda, use app as app name

* remove the sleep statement since we rely on the ticker

examples/prometheus-federation/README.md

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+# Prometheus Federation Endpoint Example
+
+This example showcases how the agent works with Splunk Enterprise and an existing Prometheus deployment.
+
+The example runs as a Docker Compose deployment. The collector can be configured to send various metrics to Splunk Enterprise.
+
+Splunk is configured to receive data from the OpenTelemetry Collector using the HTTP Event collector. To learn more about HEC, visit [our guide](https://dev.splunk.com/enterprise/docs/dataapps/httpeventcollector/).
+
+To deploy the example, check out this git repository, open a terminal and in this directory type:
+```bash
+$> docker-compose up --build
+```
+
+Splunk will become available on port 18000. You can login on [http://localhost:18000](http://localhost:18000) with `admin` and `changeme`.
+
+Once logged in, visit the [analytics workspace](http://localhost:18000/en-US/app/search/analytics_workspace) to see which metrics are sent by the OpenTelemetry Collector.
+
+Additionally, you can consult the [Prometheus UI](http://localhost:9090) to see the metric data collected from the sample go program.
+
+# Diagram of the deployment
+
+![Diagram](diagram.png)

examples/prometheus-federation/docker-compose.yml

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+version: "3"
+services:
+  # Sample Go application producing counter metrics.
+  prom-counter:
+    container_name: prom-counter
+    build:
+      context: prom-counter
+    restart: always
+  # The Prometheus server:
+  prometheus:
+    image: prom/prometheus
+    container_name: prometheus
+    ports:
+      - 9090:9090
+    volumes:
+      - ./prometheus.yml:/etc/prometheus/prometheus.yml
+  # Splunk Enterprise server:
+  splunk:
+    image: splunk/splunk:latest
+    container_name: splunk
+    environment:
+      - SPLUNK_START_ARGS=--accept-license
+      - SPLUNK_HEC_TOKEN=00000000-0000-0000-0000-0000000000000
+      - SPLUNK_PASSWORD=changeme
+    ports:
+      - 18000:8000
+    healthcheck:
+      test: ['CMD', 'curl', '-f', 'http://localhost:8000']
+      interval: 5s
+      timeout: 5s
+      retries: 20
+    volumes:
+      - ./splunk.yml:/tmp/defaults/default.yml
+      - /opt/splunk/var
+      - /opt/splunk/etc
+  # OpenTelemetry Collector
+  otelcollector:
+    image: quay.io/signalfx/splunk-otel-collector:0.4.0
+    container_name: otelcollector
+    command: ["--config=/etc/otel-collector-config.yml", "--log-level=DEBUG"]
+    volumes:
+      - ./otel-collector-config.yml:/etc/otel-collector-config.yml
+    depends_on:
+      - splunk
+      - prometheus

examples/prometheus-federation/otel-collector-config.yml

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+receivers:
+    prometheus_simple:
+        collection_interval: 10s
+        # the federation endpoint:
+        # Read more about it here: https://prometheus.io/docs/prometheus/latest/federation/
+        # You can query the federation with PromQL, encoded as part of the query string.
+        endpoint: prometheus:9090
+        metrics_path: /federate
+        params:
+            match[]: '{job="counter"}'
+    otlp:
+        protocols:
+            grpc:
+
+exporters:
+    splunk_hec/metrics:
+        # Splunk HTTP Event Collector token.
+        token: "00000000-0000-0000-0000-0000000000000"
+        # URL to a Splunk instance to send data to.
+        endpoint: "https://splunk:8088/services/collector"
+        # Optional Splunk source: https://docs.splunk.com/Splexicon:Source
+        source: "app:metrics"
+        # Optional Splunk source type: https://docs.splunk.com/Splexicon:Sourcetype
+        sourcetype: "prometheus"
+        # Splunk index, optional name of the Splunk index targeted.
+        index: "metrics"
+        # Maximum HTTP connections to use simultaneously when sending data. Defaults to 100.
+        max_connections: 20
+        # Whether to disable gzip compression over HTTP. Defaults to false.
+        disable_compression: false
+        # HTTP timeout when sending data. Defaults to 10s.
+        timeout: 10s
+        # Whether to skip checking the certificate of the HEC endpoint when sending data over HTTPS. Defaults to false.
+        # For this demo, we use a self-signed certificate on the Splunk docker instance, so this flag is set to true.
+        insecure_skip_verify: true
+
+processors:
+    batch:
+    queued_retry:
+
+extensions:
+    health_check:
+    pprof:
+      endpoint: :1888
+    zpages:
+      endpoint: :55679
+
+service:
+    extensions: [pprof, zpages, health_check]
+    pipelines:
+      metrics:
+        receivers: [prometheus_simple]
+        processors: [batch, queued_retry]
+        exporters: [splunk_hec/metrics]

examples/prometheus-federation/prom-counter/Dockerfile

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+FROM golang:1.14-stretch
+
+WORKDIR /go/src/app
+
+COPY go.mod .
+COPY main.go .
+
+RUN go get
+
+RUN go build
+
+CMD /go/src/app/app

examples/prometheus-federation/prom-counter/go.mod

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+module app
+
+go 1.14
+
+require (
+	go.opentelemetry.io/otel v0.15.0
+	go.opentelemetry.io/otel/exporters/metric/prometheus v0.15.0
+	go.opentelemetry.io/otel/exporters/otlp v0.15.0
+	go.opentelemetry.io/otel/sdk v0.15.0
+	go.uber.org/zap v1.16.0
+)