Precise allowed return type for EventMsg Values in custom plugin

[moutyque]

Hello

From the doc and the source code the return object from a plugin is:

type EventMsg struct {
	Name      string                 `json:"name,omitempty"`
	Timestamp int64                  `json:"timestamp,omitempty"`
	Tags      map[string]string      `json:"tags,omitempty"`
	Values    map[string]interface{} `json:"values,omitempty"`
	Deletes   []string               `json:"deletes,omitempty"`
}

I was expected to be able to return any thing as a value however it breaks compatibility with based gnmic code.

I suggest to add information in the doc to restrict type as base type (int,float,string)