Aggregate metrics

[pditommaso]

The goal of this issue is to extend the v1alpha2 metrics endpoint so that it returns aggregate metrics when no date or org parameter is specified.

In this case, it should return the value of all <metrics>/o/<organization> keys and the total for all for them for example having

/pulls/o/foo: 1
/pulls/o/bar: 5

it should return

{
  "metrics": "pulls",
  "count": 6,
  "orgs": {
    "foo": 1,
    "bar": 5
  }
}