chore: update prometheus exporter readme with usage and links (#521)

* chore: update prometheus exporter readme with example and links

* fix: minor change

packages/opentelemetry-exporter-prometheus/README.md

@@ -4,9 +4,58 @@
 [![devDependencies][devDependencies-image]][devDependencies-url]
 [![Apache License][license-image]][license-image]
 
-OpenTelemetry Exporter Prometheus provides a metrics endpoint for Prometheus.
+The OpenTelemetry Prometheus Metrics Exporter allows the user to send collected [OpenTelemetry Metrics](https://github.com/open-telemetry/opentelemetry-js/tree/master/packages/opentelemetry-metrics) to Prometheus.
 
-**Work in progress**
+[Prometheus](https://prometheus.io/) is a monitoring system that collects metrics, by scraping exposed endpoints at regular intervals, evaluating rule expressions. It can also trigger alerts if certain conditions are met. For assistance setting up Prometheus, [Click here](https://opencensus.io/codelabs/prometheus/#0) for a guided codelab.
+
+## Installation
+
+```bash
+npm install --save @opentelemetry/metrics
+npm install --save @opentelemetry/exporter-prometheus
+```
+
+## Usage
+
+Create & register the exporter on your application.
+
+```js
+const { PrometheusExporter } = require('@opentelemetry/exporter-prometheus');
+const { Meter }  = require('@opentelemetry/metrics');
+
+// Add your port and startServer to the Prometheus options
+const options = {port: 9464, startServer: true};
+const exporter = new PrometheusExporter(options);
+
+// Register the exporter
+const meter = new Meter();
+meter.addExporter(exporter);
+
+// Now, start recording data
+const counter = meter.createCounter('metric_name');
+counter.add(10, meter.labels({ [key]: 'value' }));
+
+// Record data using Handle: It is recommended to keep a reference to the Handle instead of
+// always calling `getHandle()` method for every operations.
+const handle = counter.getHandle(meter.labels({ [key]: 'value' }));
+handle.add(10);
+
+// .. some other work
+
+// Create and record Gauge
+const gauge = meter.createGauge('metric_name1');
+gauge.set(10, meter.labels({ [key1]: 'value1' }));
+```
+
+## Viewing your metrics
+
+With the above you should now be able to navigate to the Prometheus UI at: http://localhost:9464/metrics
+
+## Useful links
+- For more information on OpenTelemetry, visit: <https://opentelemetry.io/>
+- To learn more about Prometheus, visit: https://prometheus.io/
+- For more about OpenTelemetry JavaScript: <https://github.com/open-telemetry/opentelemetry-js>
+- For help or feedback on this project, join us on [gitter][gitter-url]
 
 ## License