Add the possibility to use a custom graphite prefix

[CharlesJUDITH]

The goal of this PR is to use as a custom prefix from the remote_write and remote_read URL.

Basically the graphite path will be set like that in the Prometheus configuration:

remote_write:

• url: "http://localhost:9201/write?graphite.default-prefix=customprefix."

remote_read:

• url: "http://localhost:9201/read?graphite.default-prefix=customprefix."

If this custom prefix is set, then the default prefix will not be used.

[iksaif]

two very small comments, the rest looks good :+1: