Add a new label: serial

[costasd]

Subject, filename or issuer might remain the same across certificate issuances, say, in renewals.

Serial on the other hand, is an attribute that has quite better chances of changing every time a certificate changes.

So, let's expose it as a label to make in-place renewals a detectable event.

Fixes #71

- What I did

• Exposed serial as another label

- How I did it

• Registered the new label
• Used the value of x509.Certificate's SerialNumber to populate label's value

- How to verify it

• curl'ing /metrics will now contain serial as a label and a large number as its label's value.

- Description for the CHANGELOG We now emit each certificate's serial number as a label

[amimof]

@costasd Thanks for contributing. This change looks good to me. Please rebase into master and force push.

[costasd]

Done, thanks!