Add LocalStoreDrain to use in tests

LocalStoreDrain will aggregate the metrics that normally would be logged by LogDrain in a map.

```
Count("user.signup", 1)
Count("user.signup", 1)
Count("user.signup", 2)
```

will generate the following map:
```
map[user.signup:map[count#user.signup=1:2 count#user.signup=2:1]]
```