Add example config and fix stat update

[stephen-soltesz]

This change includes two bug fixes.

• bug1: the cached stat was not updated when a modification was discovered.
  • now, when files are updated, the new stat is cached.
• bug2: prometheus collector registration failures cannot be recovered.
  • now, when a registration fails, the process exists, and the runtime must restart it.

These bugs prevented our deployment of the latest bqx version in M-Lab. These fixes should unblock that. And, with this change, we should be able to deploy query updates, have the changes be noticed, and automatically updated.

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 100

• 24 of 24 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 84:	0.0%
Covered Lines:	260
Relevant Lines:	260

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💛 - Coveralls

[stephen-soltesz]

Unblocks: m-lab/prometheus-support#734
Fixes: #25

[nkinkade]

Reviewed 2 of 7 files at r1, 2 of 5 files at r2.
Reviewable status: 0 of 1 LGTMs obtained