* 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>