A linter (vet) found the following bug in the code:

> daemon/metrics.go:124::error: range variable p captured by func literal (vet)

Here a variable p is used in an async fashion by goroutine, and most
probably by the time of use it is set to the last element of a range.

For example, the following code

```go
for _, c := range []string{"here ", "we ", "go"} {
go func() {
fmt.Print(c)
}()
}
```

will print `gogogo` rather than `here we go` as one would expect.

Fixes: 0e8e8f0f31 ("Add support for metrics plugins")
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>