Go application instrumentation guide (#1121)

* Add new Go application guide
Signed-off-by: lucperkins <lucperkins@gmail.com>

* Address @brian-brazil comments
Signed-off-by: lucperkins <lucperkins@gmail.com>

* Move var() block next to init() function
Signed-off-by: lucperkins <lucperkins@gmail.com>

* Use promauto for example gauge
Signed-off-by: lucperkins <lucperkins@gmail.com>

* Switch example from gauge to counter
Signed-off-by: lucperkins <lucperkins@gmail.com>

* Fix Go code example indentation issue
Signed-off-by: lucperkins <lucperkins@gmail.com>

* Update counter name and description
Signed-off-by: lucperkins <lucperkins@gmail.com>

* Add 'See also' section for other Go libraries
Signed-off-by: lucperkins <lucperkins@gmail.com>

* Fix variable and function names
Signed-off-by: lucperkins <lucperkins@gmail.com>

content/docs/guides/go-application.md

+---
+title: Instrumenting a Go application
+---
+
+# Instrumenting a Go application for Prometheus
+
+Prometheus has an official [Go client library](https://github.com/prometheus/client_golang) that you can use to instrument Go applications. In this guide, we'll create a simple Go application that exposes Prometheus metrics via HTTP.
+
+NOTE: For comprehensive API documentation, see the [GoDoc](https://godoc.org/github.com/prometheus/client_golang) for Prometheus' various Go libraries.
+
+## Installation
+
+You can install the `prometheus`, `promauto`, and `promhttp` libraries necessary for the guide using [`go get`](https://golang.org/doc/articles/go_command.html):
+
+```bash
+go get github.com/prometheus/client_golang/prometheus
+go get github.com/prometheus/client_golang/prometheus/promauto
+go get github.com/prometheus/client_golang/prometheus/promhttp
+```
+
+## How Go exposition works
+
+To expose Prometheus metrics in a Go application, you need to provide a `/metrics` HTTP endpoint. You can use the [`prometheus/promhttp`](https://godoc.org/github.com/prometheus/client_golang/prometheus/promhttp) library's HTTP [`Handler`](https://godoc.org/github.com/prometheus/client_golang/prometheus/promhttp#Handler) as the handler function.
+
+This minimal application, for example, would expose the default metrics for Go applications via `http://localhost:2112/metrics`:
+
+```go
+package main
+
+import (
+        "net/http"
+
+        "github.com/prometheus/client_golang/prometheus/promhttp"
+)
+
+func main() {
+        http.Handle("/metrics", promhttp.Handler())
+        http.ListenAndServe(":2112", nil)
+}
+```
+
+To start the application:
+
+```bash
+go run main.go
+```
+
+To access the metrics:
+
+```bash
+curl http://localhost:2112/metrics
+```
+
+## Adding your own metrics
+
+The application [above](#how-go-exposition-works) exposes only the default Go metrics. You can also register your own custom application-specific metrics. This example application exposes a `myapp_processed_ops_total` [counter](/docs/concepts/metric_types/#counter) that counts the number of operations that have been processed thus far. Every 2 seconds, the counter is incremented by one.
+
+```go
+package main
+
+import (
+        "net/http"
+        "time"
+
+        "github.com/prometheus/client_golang/prometheus"
+        "github.com/prometheus/client_golang/prometheus/promauto"
+        "github.com/prometheus/client_golang/prometheus/promhttp"
+)
+
+func recordMetrics() {
+        go func() {
+                for {
+                        opsProcessed.Inc()
+                        time.Sleep(2 * time.Second)
+                }
+        }()
+}
+
+var (
+        opsProcessed = promauto.NewCounter(prometheus.CounterOpts{
+                Name: "myapp_processed_ops_total",
+                Help: "The total number of processed events",
+        })
+)
+
+func main() {
+        recordMetrics()
+
+        http.Handle("/metrics", promhttp.Handler())
+        http.ListenAndServe(":2112", nil)
+}
+```
+
+To run the application:
+
+```bash
+go run main.go
+```
+
+To access the metrics:
+
+```bash
+curl http://localhost:2112/metrics
+```
+
+In the metrics output, you'll see the help text, type information, and current value of the `myapp_processed_ops_total` counter:
+
+```
+# HELP myapp_processed_ops_total The total number of processed events
+# TYPE myapp_processed_ops_total counter
+myapp_processed_ops_total 5
+```
+
+You can [configure](/docs/prometheus/latest/configuration/configuration/#<scrape_config>) a locally running Prometheus instance to scrape metrics from the application. Here's an example `prometheus.yml` configuration:
+
+```yaml
+scrape_configs:
+- job_name: myapp
+  scrape_interval: 10s
+  static_configs:
+  - targets:
+    - localhost:2112
+```
+
+## Other Go client features
+
+In this guide we covered just a small handful of features available in the Prometheus Go client libraries. You can also expose other metrics types, such as [gauges](https://godoc.org/github.com/prometheus/client_golang/prometheus#Gauge) and [histograms](https://godoc.org/github.com/prometheus/client_golang/prometheus#Histogram), [non-global registries](https://godoc.org/github.com/prometheus/client_golang/prometheus#Registry), functions for [pushing metrics](https://godoc.org/github.com/prometheus/client_golang/prometheus/push) to Prometheus [PushGateways](/docs/instrumenting/pushing/), bridging Prometheus and [Graphite](https://godoc.org/github.com/prometheus/client_golang/prometheus/graphite), and more.
+
+## Summary
+
+In this guide, you created two sample Go applications that expose metrics to Prometheus---one that exposes only the default Go metrics and one that also exposes a custom Prometheus counter---and configured a Prometheus instance to scrape metrics from those applications.