cmd/prometheus: buffer signal chans
cmd/prometheus: buffer signal chans

According to the GoDoc for os.Signal [0]:

> Package signal will not block sending to c: the caller must ensure that
> c has sufficient buffer space to keep up with the expected signal rate.
> For a channel used for notification of just one signal value, a buffer
> of size 1 is sufficient.

[0] https://golang.org/pkg/os/signal/#Notify

Signed-off-by: Lucas Serven <ba02f21e1450c83731389a3d53229d9e401bc16b@gmail.com>

GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,prometheus/prometheus,keegancsmith/prometheus,alileza/prometheus,kangwoo/prometheus,kangwoo/prometheus,Gouthamve/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,Gouthamve/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,kangwoo/prometheus,alileza/prometheus,Gouthamve/prometheus,prometheus/prometheus,Gouthamve/prometheus,prometheus/prometheus,alileza/prometheus,alileza/prometheus,kangwoo/prometheus