Create documentation on what is a metric

docs/metrics/index.md

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+# Metrics
+
+This guide aims to explain basics about metrics.
+
+## What are metrics
+
+When you do monitoring on your services, servers, APIs or others, you can get
+raw data. These datas may be a response time to a request, the number of queries
+handled in a minute, etc.
+
+The metrics are these raw datas. Using the [Cachet's API][1] you can send the datas
+about what you are monitoring to Cachet.
+
+
+## What can do metrics for you
+
+Having good metrics to show may be great for customers or partners.  
+
+You have a big webservice that is under pressure? So it's important to have a
+short response time. A metric could show to your users that the webservice is
+responding fast!  
+Imagine, you have a metric named "Response time". Every 10 seconds you call your
+webservice, and send the response time to the Cachet's API, in the metric. On
+your status page you'll be able to see the average response time for a minute
+for example.
+
+Doing so, your users would see that during the last 10 minutes your response
+time was worst than previously, and it begins to being better.
+
+
+
+[1]: api-documentation.md