Add metric type documentation.

content/concepts/automatic.md

 ---
 title: Automatic Labels and Synthetic Metrics
-sort_rank: 5
+sort_rank: 2
 ---
 
 # Automatic Labels and Synthetic Metrics

content/concepts/metric_types.md

+---
+title: Metric Types
+sort_rank: 1
+---
+
+# Metric Types
+
+Prometheus offers three core metric types:
+
+  * Counters
+  * Gauges
+  * Summaries
+
+Each type is useful for a different purpose, so it is important to use the
+right one for the right job.
+
+Metric types are currently only differentiated in the client libraries (to
+enable APIs tailored to the usage of the specific types) and in the wire
+protocol. The Prometheus server does not yet persist and make use of the type
+information after ingesting samples. This may change in the future, however.
+
+## Counter
+
+A _counter_ is a cumulative metric that represents a single numerical value
+that only ever goes up. That implies that it cannot be used to count items
+whose number can also go down, e.g. the number of currently running goroutines.
+Those "counts" are represented by gauges.
+
+A counter is typically used to count requests served, tasks completed, errors
+occurred, etc.
+
+Client library usage documentation for counters:
+
+   * [Go](http://godoc.org/github.com/prometheus/client_golang/prometheus#Counter)
+   * [Java](https://github.com/prometheus/client_java/blob/master/client/src/main/java/io/prometheus/client/metrics/Counter.java)
+   * [Java (simple client)](https://github.com/prometheus/client_java/blob/master/simpleclient/src/main/java/io/prometheus/client/Counter.java)
+   * [Ruby](https://github.com/prometheus/client_ruby#counter)
+
+## Gauge
+
+A _gauge_ is a metric that represents a single numerical value that can
+arbitrarily go up and down.
+
+Gauges are typically used for measured values like temperatures or current
+memory usage, but also "counts" that can go up and down, like the number of
+running goroutines.
+
+Client library usage documentation for gauges:
+
+   * [Go](http://godoc.org/github.com/prometheus/client_golang/prometheus#Gauge)
+   * [Java](https://github.com/prometheus/client_java/blob/master/client/src/main/java/io/prometheus/client/metrics/Gauge.java)
+   * [Java (simple client)](https://github.com/prometheus/client_java/blob/master/simpleclient/src/main/java/io/prometheus/client/Gauge.java)
+   * [Ruby](https://github.com/prometheus/client_ruby#gauge)
+
+## Summaries
+
+A _summary_ samples observations (usually things like request durations) over
+sliding windows of time and provides instantaneous insight into their
+distributions, frequencies, and sums.
+
+A summary reports the following information:
+
+  * streaming **quantile values** of observed events, exposed as `<basename>{quantile="<quantile label>"}`
+  * the **total sum** of all observed values, exposed as `<basename>_sum`
+  * the **count** of events that have been observed, exposed as `<basename>_count`
+
+This is quite convenient, for if you are interested in tracking latencies of an
+operation in real time, you get three types of information reported for free
+with one metric.
+
+A typical use-case is the observation of request latencies or response sizes.
+
+Client library usage documentation for summaries:
+
+   * [Go](http://godoc.org/github.com/prometheus/client_golang/prometheus#Summary)
+   * [Java](https://github.com/prometheus/client_java/blob/master/client/src/main/java/io/prometheus/client/metrics/Summary.java)
+   * [Java (simple client)](https://github.com/prometheus/client_java/blob/master/simpleclient/src/main/java/io/prometheus/client/Summary.java)
+   * [Ruby](https://github.com/prometheus/client_ruby#summary)