Feature/custom metrics

[henrod]

Possibility to add custom metrics on starting application. The danger in this is that a wrong config will cause the app to crash because the prometheus lib crashes.

[coveralls]

Pull Request Test Coverage Report for Build 635

• 9 of 75 (12.0%) changed or added relevant lines in 4 files are covered.
• 5 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.9%) to 72.735%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
app.go	8	9	88.89%
config/config.go	1	4	25.0%
metrics/models.go	0	8	0.0%
metrics/prometheus_reporter.go	0	54	0.0%

Files with Coverage Reduction	New Missed Lines	%
agent/agent.go	2	81.79%
cluster/etcd_service_discovery.go	3	78.14%

Totals
Change from base Build 623:	-0.9%
Covered Lines:	4055
Relevant Lines:	5575

---

💛 - Coveralls

[andrehp]

Could you describe a little better how to use the custom metrics? The configuration values are not very clear for me

[henrod]

I don't know if doc is the better choice for this, maybe add an example?

[andrehp]

Yeah, I guess an example would already help

[henrod]

@andrehp I made an example

[andrehp]

Nice, could you just add a small paragraph to the docs mentioning custom metrics?

https://github.com/topfreegames/pitaya/blob/master/docs/features.md#monitoring

[henrod]

@andrehp do you think this is enough?

[andrehp]

Yeah, LGTM

I'm gonna merge this, good job!