Added info about schema to the Metricbeat dev guide (#2057)

metricbeat/docs/developer-guide/create-metricset.asciidoc

@@ -185,6 +185,73 @@ For this kind of data, you can use the following `Fetch` interface:
 The only difference between this and the previous example is that the second example returns `[]common.MapStr`.
 Metricbeat will add the same timestamp to all the events in the list to make it possible to correlate the events.
 
+[float]
+==== Parsing and normalizing fields
+
+In Metricbeat we aim to normalize the metric names from all Metricsets to
+respect a common {libbeat}/event-conventions.html[set of conventions]. This
+makes it easy for users to find and interpret metrics. To simplify parsing,
+converting, renaming, and restructuring of the object read from the monitored
+system to the Metricbeat format, we have created the
+https://godoc.org/github.com/elastic/beats/metricbeat/schema[schema] package
+that allows to declaratively define transformations.
+
+For example, assuming this input object:
+
+[source,go]
+----
+input := map[string]interface{}{
+	"testString":     "hello",
+	"testInt":        "42",
+	"testBool":       "true",
+	"testFloat":      "42.1",
+	"testObjString":  "hello, object",
+}
+----
+
+And the requirement to transform it into this one:
+
+[source,go]
+----
+common.MapStr{
+	"test_string": "hello",
+	"test_int":    int64(42),
+	"test_bool":   true,
+	"test_float":  42.1,
+	"test_obj": common.MapStr{
+		"test_obj_string": "hello, object",
+	},
+}
+----
+
+You can use the following code to make the transformations:
+
+[source,go]
+----
+import (
+	s "github.com/elastic/beats/metricbeat/schema"
+	c "github.com/elastic/beats/metricbeat/schema/mapstrstr"
+)
+
+var (
+	schema = s.Schema{
+		"test_string": c.Str("testString"),
+		"test_int":    c.Int("testInt"),
+		"test_bool":   c.Bool("testBool"),
+		"test_float":  c.Float("testFloat"),
+		"test_obj": s.Object{
+			"test_obj_string": c.Str("testObjString"),
+		},
+	}
+)
+
+func eventMapping(input map[string]interface{}) common.MapStr {
+	return schema.Apply(input)
+}
+----
+
+In the above example, note that it is possible to create the schema object once
+but to apply to all events.
 
 [float]
 === Configuration File

metricbeat/schema/doc.go

@@ -1,8 +1,8 @@
 /*
-Package schema contains helper utilities to parse, convert, rename and reorganize data
+Package schema contains helper utilities to parse, convert, rename, and reorganize data
 from the format returned by the various monitored systems to the data model used by
 Metricbeat.
 
-See the unit tests in the mapstrstr and mapstriface for usage examples.
+See the docs in the mapstrstr and mapstriface sub-packages for usage examples.
 */
 package schema