MetricType decoration to be consumed by the web api

Hi,
in the current branch for the metrics feature, there is a mechanism to provide only the relevant data to the webapi, while delivering all the metrics fields to the store. This is archived by something like

``` ruby
def to_hash
{
"namespaces" => namespaces,
"key" => key,
"type" => type,
"value" => value
}
end

def to_json_data
value
end
```

And then using JSON serialization to provide all necessary fields for the web api. This is doing it's work, but is an obscure method to select only relevant data.

I propose we introduce a decorator that transform this class by only providing the relevant data (aka minimal data). This will allow us remove the usage of `to_json_data` method, only required by jrjackson and allow us to have a more clever code to understand in the long run.

Start by reading the MetricType serialization methods shown in the issue, especially to_hash and to_json_data, and review how JSON serialization and jrjackson consume them. Determine the decorator's scope and how removing to_json_data would preserve the web API's minimal data while still exposing all metrics fields to the store.

Written by the indexing model from the issue text.