Add prometheus metric for certificate ready condition
Add prometheus metric for certificate ready condition

Signed-off-by: Michael Tsang <12c44a905ab68c8326f7d2c4e35a882987901fa1@jetstack.io>
Co-authored-by: Aleksandr Seleznev <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@slznv.net>

jetstack-experimental/cert-manager,jetstack/cert-manager,cert-manager/cert-manager,jetstack/cert-manager,jetstack-experimental/cert-manager,jetstack/cert-manager,cert-manager/cert-manager,jetstack-experimental/cert-manager,cert-manager/cert-manager