Add unique label validation (prometheus#263)

* Add unique label validation

Originally created as a PR in the
[client_golang](prometheus/client_golang#812)
repo however was suggested to move it here.

Signed-off-by: Jarod Watkins <[email protected]>

Extend version.Print with platform info (prometheus#258)

* Extend version.Print with platform info

Signed-off-by: Daniil Rutskiy <[email protected]>

config: add SetDirectory (prometheus#250)

Signed-off-by: Andy Bursavich <[email protected]>