Initialize jobs total metric on startup.

[stephen-soltesz]

Recently, we have observed alerts like https://github.com/m-lab/dev-tracker/issues/724 due to longer datatype processing times and existing short alert wait times. We want to know quickly whether the gardener service is down. So, this change initializes the gardener_jobs_total metric at startup so that it exists without interfering with existing alerts.

---

This change is 

[coveralls]

Pull Request Test Coverage Report for Build 3522

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 74.117%

---

Totals
Change from base Build 3510:	0.01%
Covered Lines:	1532
Relevant Lines:	2067

---

💛 - Coveralls