* remote_write: allow passing along custom HTTP headers

Signed-off-by: Nandor Kracser <bonifaido@gmail.com>

* add warning

Signed-off-by: Nandor Kracser <bonifaido@gmail.com>

* remote_write: add header valadtion

Signed-off-by: Nandor Kracser <bonifaido@gmail.com>

* extend tests for bad remote write headers

Signed-off-by: Nandor Kracser <bonifaido@gmail.com>

* remote_write: add note about the authorization header

Signed-off-by: Nandor Kracser <bonifaido@gmail.com>