Docs improvements

docs/content/doc/development/metrics.md

@@ -0,0 +1,48 @@
+---
+date: "2019-02-12:00:00+02:00"
+title: "Metrics"
+draft: false
+type: "doc"
+menu:
+  sidebar:
+    parent: "development"
+---
+
+# Metrics
+
+Metrics work by exposing a `/metrics` endpoint which can then be accessed by prometheus.
+
+To keep the load on the database minimal, metrics are stored and updated in redis.
+The `metrics` package provides several functions to create and update metrics.
+
+{{< table_of_contents >}}
+
+## Exposing New Metrics
+
+First, define a `const` with the metric key in redis. This is done in `pkg/metrics/metrics.go`.
+
+To expose a new metric, you need to register it in the `init` function inside of the `metrics` package like so:
+
+{{< highlight golang >}}
+// Register total user count metric
+promauto.NewGaugeFunc(prometheus.GaugeOpts{
+    Name: "vikunja_team_count", // The key of the metric. Must be unique.
+    Help: "The total number of teams on this instance", // A description about the metric itself.
+}, func() float64 {
+    count, _ := GetCount(TeamCountKey) // TeamCountKey is the const we defined earlier.
+    return float64(count)
+})
+{{< /highlight >}}
+
+Then you'll need to set the metrics initial value on every startup of vikunja.
+This is done in `pkg/routes/routes.go` to avoid cyclic imports.
+If metrics are enabled, it checks if a redis connection is available and then sets the initial values.
+A convenience function is available if the metric is based on a database struct.
+
+Because metrics are stored in redis, you are responsible to increase or decrease these based on criteria you define.
+To do this, use `metrics.UpdateCount(value, key)` where `value` is the amount you want to cange it (you can pass
+negative values to decrease it) and `key` it the redis key used to define the metric.
+
+## Using it
+
+A Prometheus config with a Grafana template is available at [our git repo](https://git.kolaente.de/vikunja/monitoring).