Report the certificate expiration as a prometheus metric.

[clokep]

This reports the epoch time (in seconds) of the APNs certificate expiry as a prometheus metric. The metric is updated each time the certificate is loaded (so each time sygnal is started).

pyOpenSSL becomes a direct dependency instead of just a transient one via aioapns.

Fixes #103

[clokep]

I think this is ready, but leaving as a draft until I get confirmation that seconds-until-expiration is the proper way to report this.

[clokep]

Updated this based on discussion on the issue. This should be ready to go now! I should note that I wasn't fully able to test this since I don't have a valid certificate for Sygnal.

[richvdh]

looks generally sensible, but a couple of thoughts

[clokep]

Thanks @richvdh! I made some updates from your feedback. The timezone handling is a bit weird, but cryptography says:

A naïve datetime representing the end of the validity period for the certificate in UTC. This value is inclusive.

And the timestamp() docs say:

Note:
There is no method to obtain the POSIX timestamp directly from a naive datetime instance representing UTC time. If your application uses this convention and your system timezone is not set to UTC, you can obtain the POSIX timestamp by supplying tzinfo=timezone.utc:

timestamp = dt.replace(tzinfo=timezone.utc).timestamp()

or by calculating the timestamp directly:

timestamp = (dt - datetime(1970, 1, 1)) / timedelta(seconds=1)

I believe I've combined these two pieces of info correctly! :)

[richvdh]

lgtm