Support for pluggable metric reporting

[leviramsey]

References #80

This doesn't do the Cinnamon bit (how that bit would work is a mystery to me), but at least this would allow users to plug in the official CloudWatch MetricsPublisher or manually setup Cinnamon metrics.

Roughly used the Akka Circuit Breaker instrumentation as a guide.

[pvlugter]

LGTM

[pvlugter]

Should we have a built-in provider for the CloudWatch metrics publisher?

[pvlugter]

And would be useful to document this.

[leviramsey]

Should we have a built-in provider for the CloudWatch metrics publisher?

Not sure... if we were to have support for full config (since it looks to have a fairly full-featured builder), then yes; otherwise (like if it's just the defaults), then I don't know how useful it would actually be.

Will write up some docs tonight.

[leviramsey]

NB: I realized that metricProviderFor was a terrible name... metricPublisherFor better captures what's actually being provided.

(Of course, I suppose the interface should be MetricPublisherProvider, but something about that name just doesn't feel right)

[leviramsey]

I almost think a CloudWatch provider (and even this interface, since it's not DynamoDB specific) should be part of a general AWS support extension. But for the time being, this is "enough".

[patriknw]

looking good