promhttp: Check validity of method and code label values (prometheus#962

) (prometheus#987)

* Check validity of method and code label values

Signed-off-by: Kemal Akkoyun <kakkoyun@gmail.com>

* Use more flexibly functional option pattern for configuration

Signed-off-by: Kemal Akkoyun <kakkoyun@gmail.com>

* Update documentation

Signed-off-by: Kemal Akkoyun <kakkoyun@gmail.com>

* Simplify

Signed-off-by: Kemal Akkoyun <kakkoyun@gmail.com>

* Fix inconsistent method naming

Signed-off-by: Kemal Akkoyun <kakkoyun@gmail.com>

Co-authored-by: Kemal Akkoyun <kakkoyun@users.noreply.github.com>

Cut v1.12.1 (prometheus#978)

* Cut v1.12.1

Signed-off-by: Kemal Akkoyun <kakkoyun@gmail.com>

* Apply review suggestions

Signed-off-by: Kemal Akkoyun <kakkoyun@gmail.com>

Cut v1.12.0 (prometheus#966)

* Cut v1.12.0

Signed-off-by: Kemal Akkoyun <kakkoyun@gmail.com>

* Bump the day

Signed-off-by: Kemal Akkoyun <kakkoyun@gmail.com>