Prometheus integration.

[bgalehouse]

We should provide a default implementation of stats.Collector
https://github.com/google/fleetspeak/blob/master/fleetspeak/src/server/stats/collector.go
Which integrates with Prometheus.

[tsehori]

We should export another metric, ingestedPayloadSize, which will be accumulated in MessageIngested. A similar metric exists, messagesSavedSize (it is possible in this case as the parameter savedPayloadBytes is passed from manager.go), but it is not the case when parameters are passed to MessageIngested (neither when it is backlogged and when it is not.

[tsehori]

@bgalehouse do you think we can resolve this issue, or are there any other enhancements or improvements regarding the Prometheus stats collector?

[mbushkov]

This is resolved now. Prometheus implementation was implemented.