[dev guide] Add docs on optional schema arguments

docs/devguide/create-metricset.asciidoc

@@ -220,7 +220,7 @@ common.MapStr{
 }
 ----
 
-You can use the following code to make the transformations:
+You can use the schema package to transform the data, and optionally mark some fields in a schema as required or not. For example:
 
 [source,go]
 ----
@@ -229,6 +229,36 @@ import (
 	c "github.com/elastic/beats/libbeat/common/schema/mapstrstr"
 )
 
+var (
+	schema = s.Schema{
+		"test_string": c.Str("testString", s.Required), <1>
+		"test_int":    c.Int("testInt"), <2>
+		"test_bool":   c.Bool("testBool", s.Optional), <3>
+		"test_float":  c.Float("testFloat"),
+		"test_obj": s.Object{
+			"test_obj_string": c.Str("testObjString", s.IgnoreAllErrors), <4>
+		},
+	}
+)
+
+func eventMapping(input map[string]interface{}) common.MapStr {
+	return schema.Apply(input) <5>
+}
+----
+<1> Marks a field as required.
+<2> If a field has no schema option set, it is equivalent to `Required`.
+<3> Marks the field as optional.
+<4> Ignore any value conversion error
+<5> By default, `Apply` will fail and return an error if any required field is missing. Using the optional second argument, you can specify how `Apply` handles different fields of the  schema. The possible values are:
+- `AllRequired` is the default behavior. Returns an error if any required field is missing, including fields that are required because no schema option is set.
+- `FailOnRequired`  will fail if a field explicitly marked as `required` is missing.
+- `NotFoundKeys(cb func([]string))` takes a callback function that will be called with a list of missing keys, allowing for finer-grained error handling.
+
+In the above example, note that it is possible to create the schema object once
+and apply it to all events. You can also use `ApplyTo` to add additional data to an existing `MapStr` object:
+[source,go]
+----
+
 var (
 	schema = s.Schema{
 		"test_string": c.Str("testString"),
@@ -239,15 +269,24 @@ var (
 			"test_obj_string": c.Str("testObjString"),
 		},
 	}
+
+	additionalSchema = s.Schema{
+		"second_string": c.Str("secondString"),
+		"second_int": ": c.Int("secondInt"),
+	}
 )
 
-func eventMapping(input map[string]interface{}) common.MapStr {
-	return schema.Apply(input)
-}
+	data, err := schema.apply(input)
+	if err != nil {
+		return err
+	}
+
+	if m.parseMoreData{
+		additionalSchema.ApplyTo(data, input)
+	}
+
 ----
 
-In the above example, note that it is possible to create the schema object once
-and apply it to all events.
 
 [float]
 ==== Configuration File