metrics add: add does not convert output to metrics

[skshetry]

Script to reproduce:

$ echo '{"a": 2, "b": 3}' > metrics.json
$ dvc add metrics.json
$ cat metrics.json.dvc
md5: b05807be887344fcfefdfee4172b01c1
outs:
- md5: 769e983695c69ac51f605be2b4977065
  path: metrics.json
  cache: true
  metric: false
  persist: false
$ dvc metrics add metrics.json
$ cat metrics.json.dvc
md5: b05807be887344fcfefdfee4172b01c1
outs:
- md5: 769e983695c69ac51f605be2b4977065
  path: metrics.json
  cache: true
  metric: false
  persist: false