Add Token Expiration gauge metric

[leklund]

New gauge metric: fastly_rt_token_expiration

Returns the expires_at timestamp of the FASTLY_API_TOKEN.

This addresses part of #146.

If the the metric is not in the blocklist, it will create a a TokenRecorder that has two methods: Gatherer() and Set()

• Gatherer() returns the registry with the new gauge metric that is appended to the default gatherers
• Set() fetches token data from api.fastly.com/tokens/self and sets the expiration timestamp (if it exists)
• The metric is labeled with the token_id and the user_id to help in finding the token in the Fastly UI.

Sample:

# HELP fastly_rt_token_expiration Unix timestamp of the expiration time of the Fastly API Token
# TYPE fastly_rt_token_expiration gauge
fastly_rt_token_expiration{token_id="token_id_1234",user_id="user_id_5678"} 1.721232155e+09