monitoring: export collector timeout as prometheus config

[GeorgeTsagk]

This PR exports the prometheus collector timeout as a prometheus config parameter.

With this timeout we will be able to observe values on our loadtest tapds for longer periods, as the timeout will be harder to reach.

[coveralls]

Pull Request Test Coverage Report for Build 12808537908

Details

• 0 of 8 (0.0%) changed or added relevant lines in 4 files are covered.
• 10 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.04%) to 40.954%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
monitoring/asset_balances_collector.go	0	1	0.0%
monitoring/asset_collector.go	0	1	0.0%
monitoring/db_collector.go	0	1	0.0%
monitoring/prometheus.go	0	5	0.0%

Files with Coverage Reduction	New Missed Lines	%
commitment/tap.go	2	84.32%
universe/interface.go	4	51.95%
tapgarden/caretaker.go	4	68.87%

Totals
Change from base Build 12791730946:	0.04%
Covered Lines:	26421
Relevant Lines:	64514

---

💛 - Coveralls

[Roasbeef]

How about we make this a config option? This needs to be set with the scape timeout in mind. Otherwise, it's possible that scrapes just continue to build up if the interval is << the scrape latency.